[FIX] pos_loyalty: not showing error without orderline

Steps to reproduce the error:
 1. Open a Pos
 2. Without any orderline, click on `+-` button
 3. You will see an error

issue https://github.com/odoo/odoo/issues/115642

closes odoo/odoo#117620

X-original-commit: a588bb7fbe9ad6accaa12ebf1ce59767284c4f12
Signed-off-by: Trinh Jacky (trj) <trj@odoo.com>
Signed-off-by: Pedram Bi Ria (pebr) <pebr@odoo.com>

addons/pos_loyalty/static/src/js/ProductScreen.js

@@ -22,7 +22,7 @@ patch(ProductScreen.prototype, "pos_loyalty.ProductScreen", {
         const _super = this._super;
         const selectedLine = this.currentOrder.get_selected_orderline();
         if (key === "-") {
-            if (selectedLine.eWalletGiftCardProgram) {
+            if (selectedLine && selectedLine.eWalletGiftCardProgram) {
                 // Do not allow negative quantity or price in a gift card or ewallet orderline.
                 // Refunding gift card or ewallet is not supported.
                 this.notification.add(