[11.x] Mark $queue as nullable

[timacdonald]

The $queue variable passed to the enqueueUsing method may be null when the calling code does not specify a queue to dispatch on, i.e., they want to use the default queue.

class MyJob implements ShouldQueue
{
    use Dispatchable;
}

Event::listen(fn (JobQueued $event) => dump($event->queue));

MyJob::dispatch();

// null

dispatch(function () {
    //
});

// null