Bugs around empty included[] in response

[bart-degreed]

The json:api spec defines conditions on when included must/must not occur in responses:

If a document does not contain a top-level data key, the included member MUST NOT be present either.
https://jsonapi.org/format/1.1/#document-top-level

If an endpoint supports the include parameter and a client supplies it:
The server’s response MUST be a compound document with an included key — even if that included key holds an empty array (because the requested relationships are empty).
https://jsonapi.org/format/1.1/#fetching-includes

JADNC currently never emits an empty included array, while it should in case the user passed an include query string parameter. It should also emit an empty array on /articles/1/author?include=posts when the author relationship is null.