[12.x] Type-hint the ResourceCollection::$collection property as nullable

[lorenzolosa]

The $collection property of Illuminate\Http\Resources\Json\ResourceCollection is currently type hinted as Illuminate\Support\Collection, but it can also be null:

$missingValue = new Illuminate\Http\Resources\MissingValue();
$resource = JsonResource::collection($missingValue);
$resource->collection; // this is null

This commonly happens when doing JsonResource::collection($this->whenLoaded('relation')).

Here I'm changing the type-hinting annotation to include the null value.