Tested on: MediaWiki 1.21.1
Email notifications regarding changes made by a user with no User: page display a blank instead of user's Full Name (or username).
For example:
Dear Admiral Ackbar,
The Foobar Wiki page CinnamonBun has been changed on 2013-05-31 by , see http://wiki.example.org/CinnamonBun for the current revision.
If the User: page exists, the Full Name appears:
Dear Admiral Ackbar,
The Foobar Wiki page CinnamonBun has been changed on 2013-05-31 by Princess Leia, see http://wiki.example.org/CinnamonBun for the current revision.
I haven't done rigorous testing here, so I'm not sure what happens if the User: page exists but the 'Full Name' is not set... probably good to check that case as well.
Danke.