fix: class cast exception, map to list

[enderdincer]

      List<Map<String, Object>> specMetadata =
          (List<Map<String, Object>>) spec.getOrDefault("metadata", Collections.emptyMap());

If the desired behaviour is to return an empty collection then returning an empty map is not right here. It should be wrapped in a List or should be an empty list. If spec.metadata is mandatory and shouldn't return an empty collection then it should throw an exception (the right one, not class cast exception).