Allow the default Model of a relation to be null when using withDefault with a callback

[MannikJ]

I've encountered an unexpected behavior with the withDefault method callback in Laravel when using composite keys. Although in the following snippet I'm using the Compoships package to define a relationship through composite keys, the issue lies in the core behavior of Laravel's withDefault method and not in the way the relation itself is defined. I only choose this example to show a case where the suggested change would be relevant.

    public function providerService()
    {
        return $this->belongsTo(
            ProviderService::class,
            ['service_id', 'provider_id'],
            ['service_id', 'provider_id'],
        )->withDefault(function (ProviderService $providerService, Task $task): ?ProviderService {
            if($task->provider_id === null) {
                return null;
            }

            if($task->service_id === null) {
                return null;
            }

            $providerService->service_id = $task->service_id;
            $providerService->provider_id = $task->provider_id;

            return $providerService;
        });
    }

I noticed that even if the callback within withDefault returns null, the relationship still returns a model instance. This behavior contradicts the expected outcome, as one would assume that returning null from the callback would result in a null default model for the relationship.

This is the line which is responsible to return a model instance even when the callback returns null:
https://github.com/laravel/framework/blob/fb14d4326b780a9b2087caf7f12440f45a0f385c/src/Illuminate/Database/Eloquent/Relations/Concerns/SupportsDefaultModels.php#L54-L54C71

While one might argue that withDefault ensuring a model instance in all cases is desirable, I believe that passing a callback should act as the definitive way to define the default model. Therefore, it would be more consistent and intuitive if the framework honored a null return from the callback as the absence of a default model.

[shaedrich]

Yeah, I mentioned that in #44004, but they never followed-up on that as the core team is eager to close issues on formalities.

[ghost]

From what I see in the docs, the withDefault is used to return a model. If you need to fill that model, you use the callback param. And that says it all. Do not use withDefault for nullable relations.
https://laravel.com/docs/10.x/eloquent-relationships#default-models