[FIX] point_of_sale: Ensure order exists before reading state in refund

Prior to this commit, there were scenarios where sync orders did not
contain an order, leading to a failure when reading its state. This
commit introduces a check to ensure the order exists in sync before its
state is read, thereby preventing this error.

opw-3856451

closes odoo/odoo#162943

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

addons/point_of_sale/static/src/app/screens/ticket_screen/ticket_screen.js

@@ -276,7 +276,7 @@ export class TicketScreen extends Component {
 
         const invoicedOrderIds = new Set(
             allToRefundDetails
-                .filter(detail => this._state.syncedOrders.cache[detail.orderline.orderBackendId].state === "invoiced")
+                .filter(detail => this._state.syncedOrders.cache[detail.orderline.orderBackendId]?.state === "invoiced")
                 .map(detail => detail.orderline.orderBackendId)
         );