Don't fail for GitLab commits that don't contain a commit message.
Don't fail for GitLab commits that don't contain a commit message.

The GitLab API can return an object that does not contain a 'message' field in
the case that the commit message is empty. This change prevents a crash in that
event.

Testing Done:
Made these changes to my local installation of rb.

Reviewed at https://reviews.reviewboard.org/r/7609/

brennie/reviewboard,chipx86/reviewboard,brennie/reviewboard,davidt/reviewboard,reviewboard/reviewboard,sgallagher/reviewboard,brennie/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,davidt/reviewboard,sgallagher/reviewboard,brennie/reviewboard,davidt/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,sgallagher/reviewboard,davidt/reviewboard