[FIX] point_of_sale: error in pos.config form when editing/discarding

Prior to 28762b79e3, `_enableButtons`
method is mainly called after saving changes in the record,
now, it is also called when discarding such that its argument can
become `undefined`. This causes error in pos.config form because it
uses a custom form which hooked to the said method in order to
reload the window when a module_ or group_ field is modified.

This commit prevents this error by having a fallback empty array
when `changedFields` is `undefined`.

closes odoo/odoo#62366

Signed-off-by: pimodoo <pimodoo@users.noreply.github.com>

addons/point_of_sale/static/src/js/web_overrides/pos_config_form.js

@@ -8,7 +8,7 @@ odoo.define('point_of_sale.pos_config_form', function (require) {
     var PosConfigFormController = FormController.extend({
         _enableButtons: function (changedFields) {
             let shouldReload = false;
-            for (let field of changedFields) {
+            for (let field of (changedFields || [])) {
                 if (
                     field.startsWith('module_') ||
                     field.startsWith('group_') ||