fix case where an issue may not have any messages yet

presubmit_canned_checks.py

 # Copyright (c) 2010 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.
 
 """Generic presubmit checks that can be reused by other presubmit checks."""
 
 ### Description checks
 
 def CheckChangeHasTestField(input_api, output_api):
   """Requires that the changelist have a TEST= field."""
@@ skipping 652 matching lines @@
   owner = input_api.re.escape(issue_props['owner'])
 
   # TODO(dpranke): This mimics the logic in
   # /tools/commit-queue/verifiers/reviewer_lgtm.py
   # We should share the code and/or remove the check there where it is
   # redundant (since the commit queue also enforces the presubmit checks).
   def match_reviewer(r):
     return email_regexp.match(r) and not input_api.re.match(owner, r)
 
   approvers = []
-  for m in issue_props['messages']:
+  for m in issue_props.get('messages', []):
     if 'lgtm' in m['text'].lower() and match_reviewer(m['sender']):
       approvers.append(m['sender'])
   return set(approvers)