[FIX] pos_coupon: Make sure loyalty rewards are considered as rewards

Current behavior:
When a loyalty reward was applied with a promotion program, a promotion
reward was added with a -Infinity value.

Steps to reproduce:
- Create a loyalty program with a reward of 5€ fixed amount for 50
  points
- Create a promotion program with a reward of 10% discount
- Open a pos session, select a customer (and add points to the loyalty
  program if needed).
- Add a product to the order, the 10% discount should be applied
- Now click on the reward button to apply the loyalty reward
- The loyalty reward is correctly applied, but a new promotion reward
  is added with a -Infinity value.

opw-3297817

closes odoo/odoo#127770

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