message: [emacs] Add cquery-extra-init-params
  To enable SerializeFormat::MessagePack and use .mpack cache files:
  (setq cquery-extra-init-params '(:cacheFormat 1))
diff: @@ -245,6 +245,8 @@ std::unique_ptr<IndexFile> Deserialize(SerializeFormat format,
  }
  case SerializeFormat::MessagePack: {
  + if (serialized.empty())
  + return nullptr;
  try {
  msgpack::object_handle oh =
  msgpack::unpack(serialized.data(), serialized.size());
  