Fix master zero-value build property passing.

scripts/master/factory/commands.py

 # 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.
 
 """Set of utilities to add commands to a buildbot factory (BuildFactory).
 
 All the utility functions to add steps to a build factory here are not
 project-specific. See the other *_commands.py for project-specific commands.
 """
 
@@ skipping 198 matching lines @@
     WithProperties.__init__(self, fmtstring, *args, **kwargs)
     self.transform = transform
     if not self.transform:
       self.transform = lambda x: x
 
   def getRenderingFor(self, build):
     ret = build.getProperties().asDict()
     # asDict returns key -> (value, source), so get the values, and convert
     # empty values to blank strings.
     for k in ret:
-      ret[k] = ret[k][0] or ''
+      ret[k] = ret[k][0] if ret[k][0] is not None else ''
     for k, v in self.lambda_subs.iteritems():
       ret[k] = v(build)
     return self.fmtstring % self.transform(ret)
 
 
 class FactoryCommands(object):
   # Use this to prevent steps which cannot be run on the same
   # slave from being done together (in the case where slaves are
   # shared by multiple builds).
   slave_exclusive_lock = SlaveLock('slave_exclusive', maxCount=1)
@@ skipping 1118 matching lines @@
 
   def commandComplete(self, cmd):
     out = cmd.logs['stdio'].getText()
     build_properties = re.findall('BUILD_PROPERTY ([^=]*)=(.*)', out)
     for propname, value in build_properties:
       # findall can return strings containing CR characters, remove with strip.
       self.build.setProperty(propname, value.strip(), 'Step')
 
   def getText(self, cmd, results):
     return self.describe(True) + self.messages