[14.0][FIX] product_attribute_set: Overwrite of the attribute set by category

[fkantelberg]

The option attribute_set_id on product.category is labeled to be the "Default Attribute Set" but if you change the product category of a product it will always overwrite the attribute_set_id there if the attribute set of the product isn't set at the same time. The behaviour is also different from the onchange of categ_id. This is in all versions.

The patch will only copy the attribute_set_id from the category if it isn't already set on the product.