When trying to split an artist that is already empty (eligible for cleanup), the message given is:
There are no recordings, release groups, releases or tracks credited to only $artist. If you are trying to remove $artist, please edit all artist credits at the bottom of the aliases tab and remove all existing relationships instead, which will allow ModBot to automatically remove this artist in the upcoming days.
This is obviously useless, since there are no credits nor relationships and ModBot already can automatically remove this artist. We should just say that.