[FIX] point_of_sale: use correct partner_id for invoiced orders

Current behavior:
When invoicing an order that was made in a closed PoS session, the wrong
partner was used for the invoice. And so the total due was assigned to
the wrong partner if you used the "Customer Account" payment method.

Steps to reproduce:
- Open PoS and make an order for a customer using the Customer Account
  payment method.
- Close the PoS session.
- Open the PoS session again, and invoice the order you made in the
  previous session.

Fix:
Use the commercial_partner_id of the order's partner instead of the
partner_id directly.

opw-3478670

closes odoo/odoo#135007

Signed-off-by: Vlad Stroia (vlst) <vlst@odoo.com>