[FIX] point_of_sale: use pos_order partner when not connected

Current behavior:
When using the QRCode on ticket to create an invoice, it was always
creating a new partner, even if a partner was already set on the order.

Steps to reproduce:
- Activate the QRCode on receipt feature
- Create a PoS order and create a new customer for it
- Pay the order and print the receipt
- Scan the QRCode on the receipt to create an invoice
- On the new window enter the customer informations and see the invoice
- Now go check the customers list in the backend, you will see 2
  customers have been created.

opw-3275938

closes odoo/odoo#122514

Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>