message: gltfpack: Don't output empty children array
  If a node doesn't have attached meshes and all child nodes were
  discarded because they were not referenced, we used to output an empty
  children array which is not allowed by glTF spec.
  Fixes
diff: @@ -1262,7 +1262,12 @@ void writeNode(std::string& json, const cgltf_node& node, const std::vector<Node
  }
  append(json, "]");
  }
  - if (node.children_count || !ni.meshes.empty())
  +
  + bool has_children = !ni.meshes.empty();
  + for (size_t j = 0; j < node.children_count; ++j)
  + has_children |= nodes[node.children[j] - data->nodes].keep;
  +
  + if (has_children)
  {
  comma(json);
  append(json, "\"children\":[");
  