Fix missing callback of #2400 and ignore missing author on timeslider on...

... client-side
You're right and we've done some research on it, but can't figure out why there is no author for the authorId. One time there has been an author for the authorId in the dirty-db, but it was deleted by some reason.
I can't reproduce it on another pad and it's only happening on my development machine so it's not critical.