[go_router_builder] Allow users to specify onExit as optional - #11151

The merge queue guard is a GitHub check that prevents a PR from being merged or enqueued before it is ready. It becomes green automatically when all tests pass. No manual action is required. If you suspect that this check is not working correctly, contact #hackers-infra on Discord. If you need to merge your PR without tests (a rare situation, typically an emergency), then you can use the emergency label.