User Cleanup does not remove users if their DN is gone

This seems to have been introduced in LDAP-154 : indeed the previous code did not distinguish between null results (indication on an  ldap error) and empty results (e.g. no error, and nothing found).

However the DN-lookup returns null if not found, too, as "No object at DN found" is an ldap error. The ldap search does not return an empty result list in that case.