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()