Cache the false condition in `get_guest_author_by()`

If the false condition isn't cached, `get_coauthors()` can trigger some nasty queries on every page load because `get_coauthor_by()` will look to see if there's a guest author that should override a WordPress user. If there is no guest author profile, then CAP will again check each time `get_coauthor_by()` is used.
Merge pull request #116 from Automattic/cache-falsy-116

Cache the false condition in get_guest_author_by