[10.x] Update Facade::$app to nullable

[villfa]

Facade::$app can be null.
There are already some nullability checks within the same class (see swap() or resolveFacadeInstance()).

Making it explicit in the PHP blocks allows us to avoid errors with static analysis tools like PHPStan:

Parameter #1 $app of static method Illuminate\Support\Facades\Facade::setFacadeApplication() expects Illuminate\Contracts\Foundation\Application, null given.

[jose123v]

@taylorotwell this breaks PHPStan and other analysis tools on laravel 10.x,
on 8.x, 9.x, 11.x works because these don't have this change

Error: Cannot call method make() on Illuminate\Contracts\Foundation\Application|null.
 ------ --------------------------------------------------- 
  53     Cannot call method make() on                       
         Illuminate\Contracts\Foundation\Application|null.  
 ------ --------------------------------------------------- 

CC: @driesvints

[driesvints]

@jose123v can you share a full stack trace?