Gracefully handle empty query results

[allixender]

Querying for areas where no cell id indexed data is available returns a Status 204 no_data empty response.

We need to review if this is expected behaviour.

[jerstlouis]

@allixender If the request is for a collection, and the requested zone is completely outside the collection, then it would be better to return a 400 or 404.

204 only makes sense if the zone is within the collection extent.

i.e., there could be something interesting there, but there just isn't.

If the client is asking for a zone outside the collection's extent, it's a client error (4xx).