Fix topical event bug
Fix topical event bug

For some reason whitehall sends a null for the document type of a
topical event when it's featured for an organisation to the
publishing-api. This was causing the app to crash when the organisation
in collections was rendered featuring a topical event.

Zendesk tickets:
https://govuk.zendesk.com/agent/tickets/3839462
https://govuk.zendesk.com/agent/tickets/3841854

alphagov/collections,alphagov/collections,alphagov/collections,alphagov/collections