belongsToMany with nullable foreign - Laravel 5.8

in my Laravel project i get this database structure:

Products

• Id
• Name

Orders

• Id
• Total

Order_Product

• Product_id (nullable)
• Order_Id
• Details

In my Order model I make belongsToMany retaltionship with Product model:

public function products() {
     return $this->belongsToMany(Product::class)->withPivot('Details');
}

The Problem is when I try to get the Order Products Collection

$order->products();

I don't get rows with nullable product_id, Any solution please ? Thank you.

via Chebli Mohamed