[11.x] Allow implicit binding to have optional backed enums

[Neol3108]

I think this PR speaks for itself but this is an example of what wasn't possible before and now is:

<?php

use Illuminate\Support\Facades\Route;

Route::get('/settings/{tab?}', fn (?Tab $tab) => response()->json([
    'tab' => $tab?->value,
]));

enum Tab: string
{
    case Advanced = 'advanced';
    case General = 'general';
}

This doesn't contain breaking changes unless you expect 404s

[taylorotwell]

Can we have an actual integration test that defines a route like your example and ensures the proper behavior? Please mark as ready for review when done.

[Neol3108]

@taylorotwell casing was different in the two test files I changed. Stuck to the one used in that file

[taylorotwell]

Thanks