Fix relations between group elements

[Syndesi]

Relates to #163 .

Relations between group elements can not currently be returned, instead the API throws a 404 error.

https://github.com/ember-nexus/api/blob/main/src/Security/AccessChecker.php#L180

Example:

curl http://localhost/55d36907-283e-4877-aede-2c4503edee1b -H "Authorization: Bearer secret-token:INirKZbnZETk6hLD86rfrL" | jq

{
  "type": "http://localhost/error/404/not-found",
  "title": "Not found",
  "status": 404,
  "detail": "Requested element was not found.",
  "exception": {},
}