swarmbucket: fix empty dimension value

go/src/infra/tools/swarming-transition/cmd/swarmbucketbuilders/testdata/expected.cfg

Index: go/src/infra/tools/swarming-transition/cmd/swarmbucketbuilders/testdata/expected.cfg
diff --git a/go/src/infra/tools/swarming-transition/cmd/swarmbucketbuilders/testdata/expected.cfg b/go/src/infra/tools/swarming-transition/cmd/swarmbucketbuilders/testdata/expected.cfg
index c6911d998657b7d701771f3bbca1d886c799cf88..1949c16d24c8f71f1181682c2c4650762c282c67 100755
--- a/go/src/infra/tools/swarming-transition/cmd/swarmbucketbuilders/testdata/expected.cfg
+++ b/go/src/infra/tools/swarming-transition/cmd/swarmbucketbuilders/testdata/expected.cfg
@@ -3,6 +3,7 @@ common_dimensions: "cpu:x86-64"
 common_dimensions: "pool:Chrome"
 common_recipe {
   repository: "https://chromium.googlesource.com/chromium/tools/build"
+  properties: "a:a"
 }
 
 # Keep builders sorted by category, then name.
@@ -13,6 +14,7 @@ builders {
   dimensions: "os:Ubuntu-14.04"
   recipe {
     name: "run_presubmit"
+    properties: "a:"
     properties: "repo_name:build"
     properties_j: "runhooks:true"
   }