[FIX] point_of_sale: stop showing "select customer" popup twice

Current behavior:
When gift card module is activated for a certain PoS, and you use
customer account as the payment method without selecting a customer
the popup asking you to select a customer would appear 2 times.

Steps to reproduce:
- Activate gift cards for PoS A
- Start session in PoS A
- Add some products to the order
- Don't set any customer
- Go to the payment page
- Select "Customer account" as payment method
- Click on validate
- A popup will appear asking you to select a customer
- Click "Ok"
- The popup appears again

opw-2824675

closes odoo/odoo#91149

X-original-commit: bbb05cd9246b7478bcf1bb580dbe91ba4fb5856c
Signed-off-by: Masereel Pierre <pim@odoo.com>
Signed-off-by: Engels Robin (roen) <roen@odoo.com>

addons/pos_loyalty/static/src/js/PaymentScreen.js

@@ -31,8 +31,11 @@ export const PosLoyaltyPaymentScreen = (PaymentScreen) =>
                     pointChanges[line.coupon_id] -= line.points_cost;
                 }
             }
+            if (!await this._isOrderValid(isForceValidate)) {
+                return;
+            }
             // No need to do an rpc if no existing coupon is being used.
-            if (await this._isOrderValid(isForceValidate) && (!_.isEmpty(pointChanges) || newCodes.length)) {
+            if (!_.isEmpty(pointChanges) || newCodes.length) {
                 try {
                     const {successful, payload} = await this.rpc({
                         model: 'pos.order',