Explicitly return None if address has blank station id
Explicitly return None if address has blank station id

In general, we should not be using empty string as a valid station id
but in order to insert address data in council areas where we don't
hold any stations, we will to need to intentionally insert records
with a missing station id. Lets just be super explicit about this

chris48s/UK-Polling-Stations,DemocracyClub/UK-Polling-Stations,DemocracyClub/UK-Polling-Stations,DemocracyClub/UK-Polling-Stations,chris48s/UK-Polling-Stations,chris48s/UK-Polling-Stations