At this time (2017 Dec 05th), Return Value of Sharepoint List GetItem Lookup field id which is empty equals to null.
When a flow wrote the return value back to the original ListItem, it cause an error.
Only Number is allowed to write to lookup field.
If I want to set lookup field empty, writeback value has to be '0', not 'null'.
All return values of sharepoint list getitem should be able to writeback to original listitem without any transform.
So, return Value of Sharepoint list getItem lookup field id which is empty should be '0', not 'null'.