RevDel: Avoid log_search rows with empty values for target_author_actor
During migration, RevDel may wind up being used on items where an actor
has not been assigned yet. The code creating log_search rows for
target_author_actor needs to take this into account.
Also, to clean this up on Wikimedia wikis, I've added code to
MigrateActors to delete these rows before (re-)migrating log_search
and a --tables option so a re-run can skip trying to process all the
already-processed tables (cf. T188327#4892827).