radosgw-admin: allow 'sync group pipe modify' with existing user

Fixes: https://tracker.ceph.com/issues/68645

Signed-off-by: Casey Bodley <[email protected]>

Author: cbodley

src/rgw/rgw_admin.cc

@@ -10335,7 +10335,8 @@ int main(int argc, const char **argv)
 
     if (!rgw::sal::User::empty(user)) {
       pipe->params.user = user->get_id();
-    } else if (pipe->params.mode == rgw_sync_pipe_params::MODE_USER) {
+    } else if (pipe->params.mode == rgw_sync_pipe_params::MODE_USER &&
+               pipe->params.user.empty()) {
       cerr << "ERROR: missing --uid for --mode=user" << std::endl;
       return EINVAL;
     }