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 zeux#294.

gltf/write.cpp

@@ -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\":[");