Resolving null with DocumentDocumentReferenceUserReferenceResolver should return GuestUserReference

Right now resolving null with DocumentDocumentReferenceUserReferenceResolver returns CurrentUserReference, however it doesn't really makes sense here since historically in XWiki when a DocumentReference corresponding to a user is set to null it corresponds to Guest. We even have some checkers that we use properly null for a guest user, and not XWikiGuest.
So returning a CurrentUserReference is counter productive for this resolver: it means that any consumer of the API would have to call the resolver, then check if it's CurrentUserReference and then call a second resolver to obtain guest.