unique rules, allow product id to be null in product photos

src/Model/Entity/ProductPhoto.php

@@ -10,7 +10,7 @@ use Cake\ORM\Entity;
  * ProductPhoto Entity
  *
  * @property string $id
- * @property string $product_id
+ * @property string|null $product_id
  * @property string|null $product_category_id
  * @property string|null $product_sku_id
  * @property string $photo_dir
@@ -55,6 +55,8 @@ class ProductPhoto extends Entity
         'deleted' => true,
 
         // entities
-        'product' => true,
+        'product' => false,
+        'product_sku' => false,
+        'product_category' => false,
     ];
 }