swarmbucket: fix empty dimension value

go/src/infra/tools/swarming-transition/cmd/swarmbucketbuilders/testdata/builders.pyl

 {
   "master_base_class": "Master4a",
   "master_classname": "InfraTryServer",
   "master_port": 21402,
   "master_port_alt": 41402,
   "slave_port": 31402,
   "templates": [
     "../master.chromium.infra/templates",
     "../master.chromium/templates",
   ],
@@ skipping 18 matching lines @@
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "presubmit_build",
     },
     "Build Try Recipe Test Mac": {
       "category": "Build",
       "use_remote_run": True,
       "recipe": "build_repo_real_try",
       "properties": {
         "repo_name": "build",
         "runhooks": True,
+        "a": "a",
       },
       "scheduler": None,
       "slave_pools": ["mac_slave_pool"],
       "slavebuilddir": "build_try_recipe_test",
     },
     "Build Try Recipe Test Trusty64": {
       "category": "Build",
       "use_remote_run": True,
       "recipe": "build_repo_real_try",
       "properties": {
         "repo_name": "build",
         "runhooks": True,
+        "a": "a",
       },
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "build_try_recipe_test",
     },
     "Build Try Recipe Test Win64": {
       "category": "Build",
       "use_remote_run": True,
       "recipe": "build_repo_real_try",
       "properties": {
         "repo_name": "build",
         "runhooks": True,
+        "a": "a",
       },
       "scheduler": None,
       "slave_pools": ["win64_slave_pool"],
       "slavebuilddir": "build_try_recipe_test",
     },
 
     "Depot Tools Presubmit": {
       "category": "Depot Tools",
       "recipe": "run_presubmit",
       "properties": {
         "repo_name": "depot_tools",
+        "a": "a",
       },
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "presubmit_depot_tools",
     },
 
     "Try Recipe Mac": {
       "category": "Try Recipe",
       "use_remote_run": True,
       "recipe": "try_recipe",
       "properties": {
         "repo_name": "build",
         "runhooks": True,
+        "a": "a",
       },
       "scheduler": None,
       "slave_pools": ["mac_slave_pool"],
       "slavebuilddir": "try_recipe",
     },
     "Try Recipe Trusty 64": {
       "category": "Try Recipe",
       "use_remote_run": True,
       "recipe": "try_recipe",
       "properties": {
         "repo_name": "build",
         "runhooks": True,
+        "a": "a",
       },
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "try_recipe",
     },
     "Try Recipe Win 64": {
       "category": "Try Recipe",
       "use_remote_run": True,
       "recipe": "try_recipe",
       "properties": {
         "repo_name": "build",
         "runhooks": True,
+        "a": "a",
       },
       "scheduler": None,
       "slave_pools": ["win64_slave_pool"],
       "slavebuilddir": "try_recipe",
     },
 
     "Infra Presubmit": {
       "category": "Infra",
       "recipe": "run_presubmit",
       "properties": {
         "repo_name": "infra",
         "runhooks": True,
+        "a": "a",
       },
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "presubmit_infra",
     },
     "Infra Linux Precise 32 Tester": {
       "category": "Infra",
       "use_remote_run": True,
       "recipe": "infra_repo_trybot",
       "scheduler": None,
       "slave_pools": ["precise32_slave_pool"],
       "slavebuilddir": "infra",
+      "properties": {
+        "a": "a",
+      },
     },
     "Infra Linux Trusty 64 Tester": {
       "category": "Infra",
       "use_remote_run": True,
       "recipe": "infra_repo_trybot",
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "infra",
+      "properties": {
+        "a": "a",
+      },
     },
     "Infra Mac Tester": {
       "category": "Infra",
       "use_remote_run": True,
       "recipe": "infra_repo_trybot",
       "scheduler": None,
       "slave_pools": ["mac_slave_pool"],
       "slavebuilddir": "Infra",
+      "properties": {
+        "a": "a",
+      },
     },
     "Infra Win Tester": {
       "category": "Infra",
       "use_remote_run": True,
       "recipe": "infra_repo_trybot",
       "scheduler": None,
       "slave_pools": ["win64_slave_pool"],
       "slavebuilddir": "infra",
+      "properties": {
+        "a": "a",
+      },
     },
 
     "Luci-py Presubmit": {
       "category": "Luci-py",
       "recipe": "run_presubmit",
       "properties": {
         "repo_name": "luci_py",
         "runhooks": True,
+        "a": "a",
       },
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "presubmit_lucipy",
     },
 
     "Luci-GAE Presubmit": {
       "category": "Luci-GAE",
       "use_remote_run": True,
       "recipe": "luci_gae",
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "presubmit_lucigae",
+      "properties": {
+        "a": "a",
+      },
     },
 
     "Luci-GAE Linux Trusty 64 Tester": {
       "category": "Luci-GAE",
       "use_remote_run": True,
       "recipe": "luci_gae",
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "luci-gae",
+      "properties": {
+        "a": "a",
+      },
     },
     "Luci-go Presubmit": {
       "category": "Luci-go",
       "use_remote_run": True,
       "recipe": "luci_go",
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "presubmit_lucigo",
+      "properties": {
+        "a": "a",
+      },
     },
     "Luci-go Linux Trusty 64 Tester": {
       "category": "Luci-go",
       "use_remote_run": True,
       "recipe": "luci_go",
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "luci-go",
+      "properties": {
+        "a": "a",
+      },
     },
     "Luci-go Linux Precise 32 Tester": {
       "category": "Luci-go",
       "use_remote_run": True,
       "recipe": "luci_go",
       "scheduler": None,
       "slave_pools": ["precise32_slave_pool"],
       "slavebuilddir": "luci-go",
+      "properties": {
+        "a": "a",
+      },
     },
     "Luci-go Mac Tester": {
       "category": "Luci-go",
       "use_remote_run": True,
       "recipe": "luci_go",
       "scheduler": None,
       "slave_pools": ["mac_slave_pool"],
       "slavebuilddir": "luci-go",
+      "properties": {
+        "a": "a",
+      },
     },
     "Luci-go Win Tester": {
       "category": "Luci-go",
       "use_remote_run": True,
       "recipe": "luci_go",
       "scheduler": None,
       "slave_pools": ["win64_slave_pool"],
       "slavebuilddir": "luci-go",
+      "properties": {
+        "a": "a",
+      },
     },
 
     "Recipes-py Presubmit": {
       "category": "Recipes-py",
       "recipe": "run_presubmit",
       "properties": {
         "repo_name": "recipes_py",
         "runhooks": True,
+        "a": "a",
       },
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "presubmit_recipespy",
     },
 
     "Recipe Roll Downstream Tester": {
       "category": "Recipe Roll Tryjob",
       "use_remote_run": True,
       "recipe": "recipe_roll_tryjob",
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "recipe_roll_tryjob",
+      "properties": {
+        "a": "a",
+      },
     },
 
     "gerrit-test-cq-normal": {
       "category": "Gerrit CQ",
       "use_remote_run": True,
       "recipe": "gerrit_cq_test",
       "scheduler": None,
       "slave_pools": ["trusty64_slave_pool"],
       "slavebuilddir": "gerrit-test-cq-normal",
+      "properties": {
+        "a": "a",
+      },
     },
   },
 
   "schedulers": {},
 
   "slave_pools": {
     "precise32_slave_pool": {
       "slave_data": {
         "bits": 32,
         "os": "linux",
@@ skipping 35 matching lines @@
         "os": "win",
         "version": "win7",
       },
       "slaves": [
         "vm253-m4",
         "vm712-m4",
       ],
     },
   },
 }