[8.x] Ensure attribute is Collection when casting even if value is null

[u12206050]

Currently when casting an attribute to asCollection it only casts it if the value isn't null, creating an issue when you want to start pushing or using the collection.

This PR ensures you always get a Collection but still maintains the null value in the database if the attribute is still empty and the original value was null upon saving.

Benefit: You can just use the attribute as a collection without checking if it is indeed a Collection

[GrahamCampbell]

Please add some tests.

[vdauchy]

Not sure how your code will interact with a nullable json column.
What I do to avoid your issue is to ensure your attribute is initialized, you can do that with the property:

YourModel extends Model {
    protected $attributes = [
        'you_collection_field' => '{}',
    ];
}

[u12206050]

Not sure how your code will interact with a nullable json column.
What I do to avoid your issue is to ensure your attribute is initialized, you can do that with the property:

YourModel extends Model {
    protected $attributes = [
        'you_collection_field' => '{}',
    ];
}

It will still work with nullable columns. You will still be able to set the attribute to null or just save an empty Collection.
The only issue might be if you wanted to explicitly check if it is null, then you would need to do it on the getRawOriginal($key)

[taylorotwell]

Seems like a breaking change. I don't really plan to change this behavior. Free to make your own cast if you need something different.