[7.x] Fix SendQueuedNotifications notifiables property type

[axlon]

Currently the notifiables property is typehinted as a collection, but it isn't actually (always) one. For instance when doing $request->user()->notify(new MyNotification), the notifiables property will be set to an instance of App\User.

For the sending of the notifications it does not seem to matter if its actually a collection or not, so I opted to always wrap a collection around the given notifiable(s); this allows for collection operations in notification middleware without having to do type checks.

For example:

public function handle($job, $next)
{
    if ($job->notifiables->isNotEmpty()) {
        // do something...
    }
}

Side note: I didn't know which version to target this to, because technically it's not working correctly at the moment, but I can also see this breaking people's code if they are using notification middleware.

[driesvints]

@axlon this seems to have lead to issues regarding model serialization: #32039

[axlon]

@driesvints Should we wrap it in an Eloquent collection when its a model? This would conform to the typehint and should also fix serialization

Another option would be to check for \Illuminate\Contracts\Queue\QueueableCollection, and if it is don't wrap, although this technically doesn't ensure that its actually a subclass of Collection.

The changes could also be reverted, but this would not have my preference because that could break people's projects

[driesvints]

@axlon if that fixes the problem then yes definitely.