R1:b3ac5ceb387f

Fix DifferentialGetCommitMessageConduitAPIMethod execute strlen(null)

Summary:
When iterating through the fields of a differential commit, the DifferentialGetCommitMessageConduitAPIMethod execute method explicitly allows a value to be either a string or a null. It then calls strlen upon this possibly null value.

We could replace the strlen with phutil_nonempty_string, but as the code has already eliminated variable types other than string or null, it is more efficient to explicitly check for null or ''…