[FIX] pos_restaurant: sync empty orders

* In a pos-session with table management create a new order with products
and go back to the floor-plan.
* The order is sinced to the back-end.
* Open the table again, remove all products but keep the order.
* Go back to floorplan to trigger syncing again.
* Open the table again.

behaviour before fix:
* The empty order is not loaded in the frontend anymore.
* To close the session the order has to be manually removed in the backend.

After fix:
* The order is still visible in the frontend and can be removed or finalysed as expected.

closes odoo/odoo#40165

X-original-commit: 3add8dd2
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>