Detect password usage based on token length

The client protocol module can resolve whether a password was used based
on the information the authenticators gather before authentication is
done. It uses the authentication token length as the basis on which it
makes the decision.

server/modules/protocol/MySQL/MySQLBackend/mysql_backend.c

@@ -1510,7 +1510,7 @@ static int gw_change_user(DCB *backend,
         message = create_auth_fail_str(username,
                                        backend->session->client_dcb->remote,
                                        password_set,
-                                       "",
+                                       false,
                                        auth_ret);
         if (message == NULL)
         {