Allow trychange to submit empty patch sets again.

A recent change to trychange prevents empty patches from being submitted:
https://codereview.chromium.org/184343003/diff/210001/trychange.py

parasite (used for the gyp waterfall) relies on empty patches to share nacl
trybots to build gyp at particular revisions.

BUG=None
TEST=local on master3
R=nodir@chromium.org

Review URL: https://codereview.chromium.org/228883004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@262493 0039d316-1c4b-4281-b951-d872f2087c98

trychange.py

@@ -500,7 +500,7 @@ def _TempFilename(name, contents=None):
   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