[14.0][FIX] mrp_multi_level: error with 3 step delivery.

If the destination moves do not include a MO, the mrp move end
up without a name that raises an error later. This can happen
with 3 setep delivery but also with in more cases in wich there is
a chain of stock moves.

We ensure that the variable `order_number` always have a value.

mrp_multi_level/wizards/mrp_multi_level.py

@@ -71,7 +71,7 @@ class MultiLevelMrp(models.TransientModel):
                 parent_product_id = (
                     move_dest_id.production_id.product_id or move_dest_id.product_id
                 ).id
-        else:
+        if not order_number:
             order_number = (move.picking_id or move).name
             origin = "mv"
         mrp_date = date.today()