swarmbucket: fix empty dimension value

appengine/cr-buildbucket/swarming/swarming.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """This module integrates buildbucket with swarming.
 
 A bucket config may have "swarming" field that specifies how a builder
 is mapped to a recipe. If build is scheduled for a bucket/builder
 with swarming configuration, the integration overrides the default behavior.
 
@@ skipping 284 matching lines @@
   parse_cfg_dim = lambda d: d.split(':', 1)
   bucket_dims = map(parse_cfg_dim, bucket_cfg_dims)
   builder_dims = map(parse_cfg_dim, builder_cfg_dims)
 
   # Note: dimensions must be unique.
   # Overwrite global dimensions by bucket-level dimensions,
   # then by builder-level dimensions.
   merged = {}
   for ds in (global_dims, bucket_dims, builder_dims):
     merged.update(ds)
-  return sorted({'key': k, 'value': v} for k, v in merged.iteritems())
+  return sorted({'key': k, 'value': v} for k, v in merged.iteritems() if v)
 
 
 @ndb.tasklet
 def create_task_async(build):
   """Creates a swarming task for the build and mutates the build.
 
   May be called only if is_for_swarming(build) == True.
   """
   if build.lease_key:
     raise errors.InvalidInputError(
@@ skipping 369 matching lines @@
 def _extend_unique(target, items):
   for x in items:
     if x not in target:  # pragma: no branch
       target.append(x)
 
 
 class TaskToken(tokens.TokenKind):
   expiration_sec = 60 * 60 * 24  # 24 hours.
   secret_key = auth.SecretKey('swarming_task_token', scope='local')
   version = 1