Do not set a shipment cost method for Shopify order without shipping line

Without a shipping line, there is no carrier selected and thus no shipment cost
method to set.

Closes #14976
(grafted from 313b466eb1c55ac922819a5b66dc49e081e85064)

--HG--
branch : 8.0

modules/web_shop_shopify/sale.py

@@ -836,8 +836,8 @@ class Sale_ShipmentCost(metaclass=PoolMeta):
             else:
                 if available_carriers:
                     carrier = available_carriers[0]
-                else:
-                    shipment_cost_method = None
+        if not carrier:
+            shipment_cost_method = None
         setattr_changed(self, 'carrier', carrier)
         setattr_changed(self, 'shipment_cost_method', shipment_cost_method)