Return 404 Responses for detail views

Our current backend implementations return null responses for get requests where the resource doesn't exists.

For example https://sensors.gfz-potsdam.de/backend/api/v1/contacts/{non-existing-url} like https://sensors.gfz-potsdam.de/backend/api/v1/contacts/1000000 (hope this will stay non existing in the near future as well).

I would expect that those return a 404 response.