[9.x] Ensure middleware group exists

[rodrigopedra]

Closes #42159

After PR #42004 the behavior of method Illuminate\Routing\Router@middlewareGroup was slightly changed, as before it would define a middleware group even if the group was empty, e.g. the group had an empty array of middleware.

After the aforementioned PR, a middleware group with an empty array does not actually add a corresponding middleware group to the stack, thus when running a route with that empty middleware group, the dispatcher would try to instantiate a middleware named as the group which will fail, as reported in #42159 .

This PR:

• Adds a new protected method named ensureMiddlewareGroup, which will check if a middleware group is defined, and if not will add it.
• Modify the methods: middlewareGroup, prependMiddlewareToGroup and pushMiddlewareToGroup to use the method above

To reproduce the error described in issue #42159 one could use the following steps:

1. Create a new laravel app
2. Ensure it uses Laravel version 9.10.0
3. Add an empty middleware group to the app's HTTP kernel (for example, named public)
4. Define a route that uses the middleware defined as above
5. Access the defined route through the browser

After applying the changes proposed in this PR, the route will work.

[toyi]

I'm afraid it doesn't solves #42159 because a middleware group is still not entirely replaced by using middlewareGroup, which was the case before #42004:

public function middlewareGroup($name, array $middleware)
    {
        $this->middlewareGroups[$name] = $middleware;

        return $this;
    }

Notice that $middleware is an array even if it's singular, that typo was fixed in #42004
Allowed a package to litterally replace, let's say, the api group, by another set of middlewares.

[taylorotwell]

Reverting the original PR.