git-arc: Fix find_author() for external users

When an external user submits a review in Phabricator, the find_author()
subroutine may receive the author_name and author_addr as "null" from
the JSON API with git-arc patch.

This "null" string gets tested for length by "[ -n", and returning that
is greater than zero, in other words, the shell does not understand that
"null" means nothing in this case.

Fix it by adding a guard, that sets an empty string when the content is
"null".

While here, standardize the indentation style for find_author()

Reviewed by: emaste, markj
Approved by: emaste (mentor)
Differential Revision: https://reviews.freebsd.org/D46789