[9.x] Fix getting '0' from route parameter in Authorize middleware

[bastien-phi]

Given the following :

// api.php
Route::get('sin/{angle}', [SinusController::class, 'compute'])
    ->whereNumber('angle')
    ->can('compute', [Sinus::class, 'angle']); 

// SinusPolicy
public function compute(User $user, int $angle)
{
    return 0<= $angle && $angle <= 360;
}

Calling GET api/sin/90 will correctly pass but calling GET api/sin/0 will fail in the policy because $angle is null instead of expected 0.

Problem comes from the fact that $request->route($model, null) is '0' and '0' ?: 'bar' is 'bar'.

Changing the Elvis ?: to null coalesce ?? solves the issue.