call json constructor explicitly

tiny_gltf.h

@@ -4828,7 +4828,7 @@ static void SerializeExtensionMap(ExtensionMap &extensions, json &o) {
       if (!(extIt->first.empty())) {  // name should not be empty, but for sure
         // create empty object so that an extension name is still included in
         // json.
-        extMap[extIt->first] = json({});
+        extMap[extIt->first] = json(std::initializer_list<nlohmann::detail::json_ref<nlohmann::basic_json<>>>());
       }
     }
   }