[12.x] Add conditional return type hint for Route::middleware() method.

[marcreichel]

This pull request adds a conditional return type to the Route::middleware() method based on the $middleware parameter, so PHPStan knows that the method returns the Route instance when $middleware is not null.

This is necessary for PHPStan level 7+ to be able to call Route::get('/')->middleware('auth')->name('home');, because otherwise PHPStan does not know if the Route or an array gets returned from the middleware method.