fix git-cl-try without parameters

git_cl.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 # Copyright (C) 2008 Evan Martin <martine@danga.com>
 
 """A git-command for integrating reviews on Rietveld and Gerrit."""
 
 from __future__ import print_function
@@ skipping 332 matching lines @@
 def _get_bucket_map(changelist, options, option_parser):
   """Returns a dict mapping bucket names to builders and tests,
   for triggering try jobs.
   """
   # If no bots are listed, we try to get a set of builders and tests based
   # on GetPreferredTryMasters functions in PRESUBMIT.py files.
   if not options.bot:
     change = changelist.GetChange(
         changelist.GetCommonAncestorWithUpstream(), None)
     # Get try masters from PRESUBMIT.py files.
-    return presubmit_support.DoGetTryMasters(
+    masters = presubmit_support.DoGetTryMasters(
         change=change,
         changed_files=change.LocalPaths(),
         repository_root=settings.GetRoot(),
         default_presubmit=None,
         project=None,
         verbose=options.verbose,
         output_stream=sys.stdout)
+    if masters is None:
+      return None
+    return {MASTER_PREFIX + m: b for m, b in masters.iteritems()}
 
   if options.bucket:
     return {options.bucket: {b: [] for b in options.bot}}
   if options.master:
     return {_prefix_master(options.master): {b: [] for b in options.bot}}
 
   # If bots are listed but no master or bucket, then we need to find out
   # the corresponding master for each bot.
   bucket_map, error_message = _get_bucket_map_for_builders(options.bot)
   if error_message:
@@ skipping 5015 matching lines @@
 if __name__ == '__main__':
   # These affect sys.stdout so do it outside of main() to simplify mocks in
   # unit testing.
   fix_encoding.fix_encoding()
   setup_color.init()
   try:
     sys.exit(main(sys.argv[1:]))
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)