Return consistent object from /postcode endpoint
Return consistent object from /postcode endpoint

On reflection, I think the correct behaviour here is
that 'address' should contain an empty collection
where it is n/a.

Where the expected key is an object
('polling_station', 'custom_finder')
returning an empty object {} is misleading, so I
think null is the right value in this situation.

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