Added checks to make sure $key and Auth::user() are present. Save null i...

Quick fix to prevent Exceptions when updating Revisionable models in a seed file Issue #12.

Setting the user id to null will cause issues. Calls to $history->userResponsible()->first_name will throw errors if checks are not in place. Suggest there's some sort of fallback user for this situation, or if a user is removed after making changes.