Fix gerrit_util to use non None body.
Fix gerrit_util to use non None body.

Gerrit requires Content-Length to be set for POST requests.
httplib Python library has a bug which doesn't set Content-Length
if the body is empty.

Python BUGS:
https://bugs.python.org/issue14721 (partial fix)
https://bugs.python.org/issue23539 (full fix)

The full fix only appears in 2.7.11 release changelog
(technically, it's been fixed in 2.7.10 rc1, but 2.7.10 was not
released):
https://hg.python.org/cpython/raw-file/53d30ab403f1/Misc/NEWS

R=andybons@chromium.org,szager@chromium.org
BUG=579160

Review URL: https://codereview.chromium.org/1827653002

git-svn-id: fd409f4bdeea2bb50a5d34bb4d4bfc2046a5a3dd@299459 0039d316-1c4b-4281-b951-d872f2087c98

primiano/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools,CoherentLabs/depot_tools,primiano/depot_tools