It's just an error printed but empty user should be skipped like any other non existing user.
Shouldn't the root caused be fixed instead, ie we shouldn't have an empty user?
Having empty user in a document is technically possible and has been made on purpose in this case AFAIK (saving the document with a user that can't have programming right) so the autowatchlist should support it and not print an error.
How can you send a watchlist mail if you don't know the user?
What this has to do with watchlist ? No mail is sent to this user.