swarmbucket: fix empty dimension value

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

 common_swarming_tags: "allow_milo:1"
 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.
 
 builders {
   category: "Build"
   name: "Build Presubmit (Swarming)"
   dimensions: "os:Ubuntu-14.04"
   recipe {
     name: "run_presubmit"
+    properties: "a:"
     properties: "repo_name:build"
     properties_j: "runhooks:true"
   }
 }
 
 builders {
   category: "Build"
   name: "Build Try Recipe Test Mac (Swarming)"
   dimensions: "os:Mac-10.9"
   recipe {
@@ skipping 212 matching lines @@
 builders {
   category: "Try Recipe"
   name: "Try Recipe Win 64 (Swarming)"
   dimensions: "os:Windows-7-SP1"
   recipe {
     name: "try_recipe"
     properties: "repo_name:build"
     properties_j: "runhooks:true"
   }
 }