Laravel Route Group Second Parameter?

I updated my project from laravel 5.5.* to 5.5.43... Before the update everything was ok. But after the update, I see a warning message in route/web.php.

It says: Required parameter $routes missing.

But everything is working fine. Then when I put an empty string in the second parameter warning message gone.

So my question is:

1. What should I put in the required second parameter?
2. If it's required then why everything is working fine? It should be optional.

via Chebli Mohamed