[FIX] point_of_sale: avoid overpayment if no cash method configured

Current behavior:
If the PoS has no cash method configured, and you try to make a
payment with a greater amount than the due amount, you will get an
error message. Because you cannot give money back to the customer as you
have no cash method configured.

Steps to reproduce:
- Remove Cash payment method from the PoS
- Open a PoS session
- Create an order with some products
- Pay the order with a greater amount than the due amount
- Validate the order, you will get an error message

Solution:
If the Pos has no cash method configured, and the user try to make a
payment with a greater amount than the due amount, we will automatically
set the amount to pay to the due amount and show a message to the user.

opw-3061612

closes odoo/odoo#107365

X-original-commit: ff6bb9d362aefc64770c09b3ddd741106d301af3
Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>
Signed-off-by: Engels Robin (roen) <roen@odoo.com>