Allow trychange to submit empty patch sets again.
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: bd64dd6fa6f3f0ed0c0666d1018379882b742947@262493 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools