Test sale_drop_location value before returning id

The default_drop_location method should not fail if there is no
sale_drop_location configured.

issue6690
review35221002

sale.py

@@ -78,7 +78,8 @@ class Sale:
         Config = pool.get('sale.configuration')
 
         config = Config(1)
-        return config.sale_drop_location.id
+        if config.sale_drop_location:
+            return config.sale_drop_location.id
 
     def get_drop_shipments(self, name):
         DropShipment = Pool().get('stock.shipment.drop')