[FIX] pos_coupon: remove 0€ reward from the list of rewards

Current behavior:
When you use a coupon that gives a free product combined with a promo
that gives 50% discount. The reward of 50% disappears when you add the
free product.

Steps to reproduce:
- Create 2 program with no conditions :
  - One that gives 50% discount on all products with a code
  - One that gives a free product A
- Make sure product A has a tax
- Open PoS
- Add a product with no tax to the order
- Apply the 50% discount
- Add product A
- The reward of 50% disappears

Note:
This happens because we try to apply a 50% discount on amount that is 0
wich leads to a reward with a NaN amount. Wich makes the reward for the
program disappears, because here (https://github.com/odoo/odoo/blob/15.0/addons/pos_coupon/static/src/js/coupon.js#L624)
currentMaxKey is null.

opw-3529959

closes odoo/odoo#142527

X-original-commit: 9be0955218b5584f27a75b401946a853ac2588ae
Signed-off-by: Vlad Stroia (vlst) <vlst@odoo.com>
Signed-off-by: Robin Engels (roen) <roen@odoo.com>

addons/pos_loyalty/static/tests/tours/PosLoyaltyTour.js

@@ -254,3 +254,17 @@ registry.category("web_tour.tours").add("PosLoyaltyTour7", {
             PosLoyalty.orderTotalIs("90"),
         ].flat(),
 });
+
+registry.category("web_tour.tours").add('PosLoyaltyTour8', {
+    test: true,
+    url: '/pos/web',
+    steps: () =>
+        [
+            ProductScreen.confirmOpeningPopup(),
+            ProductScreen.clickHomeCategory(),
+
+            ProductScreen.clickDisplayedProduct('Product B'),
+            ProductScreen.clickDisplayedProduct('Product A'),
+            ProductScreen.totalAmountIs('50.00'),
+        ].flat(),
+});

addons/pos_loyalty/tests/test_frontend.py

@@ -1259,3 +1259,66 @@ class TestUi(TestPointOfSaleHttpCommon):
             "PosLoyaltyPromotion",
             login="pos_user",
         )
+
+    def test_promo_with_free_product(self):
+        self.env['loyalty.program'].search([]).write({'active': False})
+        self.tax01 = self.env["account.tax"].create({
+            "name": "C01 Tax",
+            "amount": "15.00",
+        })
+        self.product_a = self.env["product.product"].create(
+            {
+                "name": "Product A",
+                "type": "product",
+                "list_price": 100,
+                "available_in_pos": True,
+                "taxes_id": [(6, 0, self.tax01.ids)],
+            }
+        )
+        self.product_b = self.env["product.product"].create(
+            {
+                "name": "Product B",
+                "type": "product",
+                "list_price": 100,
+                "available_in_pos": True,
+                "taxes_id": False,
+            }
+        )
+        self.free_product = self.env['loyalty.program'].create({
+            'name': 'Free Product A',
+            'program_type': 'promotion',
+            'trigger': 'auto',
+            'applies_on': 'current',
+            'rule_ids': [(0, 0, {
+                'reward_point_mode': 'unit',
+                'minimum_qty': 0,
+            })],
+            'reward_ids': [(0, 0, {
+                'reward_type': 'product',
+                'reward_product_id': self.product_a.id,
+                'reward_product_qty': 1,
+                'required_points': 1,
+            })],
+        })
+        self.env['loyalty.program'].create({
+            'name': 'Discount 50%',
+            'program_type': 'promotion',
+            'trigger': 'auto',
+            'applies_on': 'current',
+            'rule_ids': [(0, 0, {
+                'reward_point_mode': 'order',
+                'reward_point_amount': 1,
+            })],
+            'reward_ids': [(0, 0, {
+                'reward_type': 'discount',
+                'required_points': 1,
+                'discount': 50,
+                'discount_mode': 'percent',
+            })],
+        })
+        self.main_pos_config.open_ui()
+        self.start_tour(
+            "/pos/web?config_id=%d" % self.main_pos_config.id,
+            "PosLoyaltyTour8",
+            login="pos_user",
+        )