AD fix when search could return empty dn

rhodecode/lib/auth_ldap.py

@@ -119,6 +119,9 b' class AuthLdap(object):'
                 raise ldap.NO_SUCH_OBJECT()
 
             for (dn, _attrs) in lobjects:
+                if dn is None:
+                    continue
+
                 try:
                     server.simple_bind_s(dn, password)
                     attrs = server.search_ext_s(dn, ldap.SCOPE_BASE,