[9.x] Fix default pivot attributes

[iamgergo]

This PR attempts to fix an issue, when a custom pivot model is being used with default attributes. Related PR: #18127

---

Let's say I have the same pivot setup as the linked PR, but also, my pivot has default attributes:

class UserTeamPivot extends Pivot
{
    protected $attributes = ['permissions' => '["create_campaign"]'];

    protected $casts = ['permissions' => 'json'];
}

It works well, when just creating a new instance:

(new UserTeamPivot())->toArray(); // ['permissions' => ['create_campaign']]

But fails when creating a new pivot via the relation instance, it just simply overrides the default attributes that are defined in the pivot model's $attributes property:

$user->teams()->newPivot()->toArray(); // []

---

In my opinion, this is a bug, because if I want the default attributes to be empty, then I won't define them, but if they are defined, then they should be present in a fresh pivot instance.

Casting should be working as before, and I think this is not a BC.

[crynobone]

This should definitely includes some tests

[iamgergo]

I've added a test that covers the described issue. Let me know if something else should be added.