Allow trychange to submit empty patch sets again.

trychange.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.
 
 """Client-side script to send a try job to the try server. It communicates to
 the try server by either writting to a svn/git repository or by directly
 connecting to the server by HTTP.
 """
 
@@ skipping 482 matching lines @@
 @contextlib.contextmanager
 def _TempFilename(name, contents=None):
   """Create a temporary directory, append the specified name and yield.
 
   In contrast to NamedTemporaryFile, does not keep the file open.
   Deletes the file on __exit__.
   """
   temp_dir = tempfile.mkdtemp(prefix=name)
   try:
     path = os.path.join(temp_dir, name)
-    if contents:
+    if contents is not None:
       with open(path, 'wb') as f:
         f.write(contents)
     yield path
   finally:
     shutil.rmtree(temp_dir, True)
 
 
 @contextlib.contextmanager
 def _PrepareDescriptionAndPatchFiles(description, options):
   """Creates temporary files with description and patch.
@@ skipping 604 matching lines @@
     return 1
   except (gclient_utils.Error, subprocess2.CalledProcessError), e:
     print >> sys.stderr, e
     return 1
   return 0
 
 
 if __name__ == "__main__":
   fix_encoding.fix_encoding()
   sys.exit(TryChange(None, None, False))