[12.0][FIX] purchase_open_qty refund from dropship

[sergiocorato]

How-to replicate:

• enable dropshipping
• create a stock.picking.type with supplier picking type and source location to Partner/Vendors Subcontracting, destination location to Partner/Vendors and the picking type refund to that created in the next step
• create a stock.picking.type with supplier picking type and source location to Partner/Vendors, destination location to Partner/Vendors Subcontracting and the picking type refund to that created in the previous step
• create a purchase order with the picking type of the previous step and a dropshipping address
• confirm the purchase order and receive the products (this will send the products from vendor to subcontractor)
• make a refund of the done picking

The qty_to_receive will be negative.

With this PR the qty_to_receive will be correct.