git-arc: Try harder to get the author name and email out of phab

When patching, we use the querydiffs endpoint to get a name and email
for the revision's author. It's possible that this info isn't recorded,
in which case the results after post-processing are just "null".
However, if the diff has multiple revisions, the endpoint returns an
entry for each one, some of which may contain author info, others not.
So, the deleted code which tries to filter out "null" isn't sufficient,
since the value in question might be something like "<valid
name>\nnull".

Try to make this filtering a bit smarter to avoid generating incorrect
author info.

Reviewed by: jlduran
Reported by: des
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D51065