[8.x] Fix route groups with no prefix on PHP 8.1

[GuardsmanPanda]

In php 8.1 calling trim() on null values is deprecated and since my route collection have groups without a defined prefix, I get an error on this line of code.

Parsing in an empty string as opposed to null is the minimal change which solves the problem.

[GrahamCampbell]

Thanks for the PR. Are you able to add test coverage here too, please?

[GuardsmanPanda]

I should have some time to do that shortly, I'm also adding the failing code here in the comment so you can verify (in case I forget about this PR).

class RouteServiceProvider extends ServiceProvider {
    public function boot():void {
        $this->routes(function () {
            Route::middleware(['cookie', 'session'])->group(function() {
                Route::prefix('')->group(base_path('path/to/routes.php'));
            });
        });
    }
}

[GrahamCampbell]

cc @driesvints

[GuardsmanPanda]

@driesvints @GrahamCampbell I added a testcase which fails on 8.1 without this change.