Duser_Driver/username_exists: return null if not implemented
Duser_Driver/username_exists: return null if not implemented

false means we know it doesn't exist, null means we don't know.

Signed-off-by: Florian Pritz <753f544d2d01592750fb4bd29251b78abcfc8ecd@xinu.at>

jreinert/filebin,Bluewind/filebin,jreinert/filebin,jreinert/filebin,Bluewind/filebin,jreinert/filebin,jreinert/filebin,jreinert/filebin,Bluewind/filebin,Bluewind/filebin,Bluewind/filebin