[FIX] pos_discount: stop applying discount to tips

Current behavior:
When adding a tips to an order, then applying a global discount. The
discount is applied to the tips when it should not.

Steps to reproduce:
- Open POS
- Add a product to the order
- Add a tips to the order
- Apply a global discount
- The discount is calculated on the tips and the products

opw-3271886

closes odoo/odoo#120462

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

addons/pos_discount/static/src/js/DiscountButton.js

@@ -48,7 +48,7 @@ odoo.define('pos_discount.DiscountButton', function(require) {
                 // That is, the use case of products with more than one tax is supported.
                 let tax_ids_array = tax_ids.split(',').filter(id => id !== '').map(id => Number(id));
 
-                let baseToDiscount = order.calculate_base_amount(tax_ids_array, lines.filter(ll => !ll.reward_id));
+                let baseToDiscount = order.calculate_base_amount(tax_ids_array, lines.filter(ll => !ll.reward_id && (!this.env.pos.config.tip_product_id || ll.product.id !== this.env.pos.config.tip_product_id[0])));
 
                 // We add the price as manually set to avoid recomputation when changing customer.
                 let discount = - pc / 100.0 * baseToDiscount;