getAssociation<TModel extends TRepositoryModel>  method 
Used exclusively by the OfflineFirstAdapter. If there are no results, returns null.
Implementation
Future<List<TModel>?> getAssociation<TModel extends TRepositoryModel>(Query query) async {
  logger.finest('#getAssociation: $TModel $query');
  final results = await get<TModel>(
    query: query,
    policy: _latestGetPolicy ?? OfflineFirstGetPolicy.awaitRemoteWhenNoneExist,
  );
  if (results.isEmpty) return null;
  return results;
}