Always return a Decimal for shipment cost

Other modules rely on having a Decimal to alter the cost.

issue11688
review437341004

stock.py

@@ -140,6 +140,8 @@ class ShipmentCostMixin:
             return Currency.compute(
                 self.cost_currency_used, self.cost_used,
                 self.company.currency, round=False)
+        else:
+            return Decimal(0)
 
 
 class ShipmentOutCostMixin(ShipmentCostMixin):