message: fix: should not serailize undefined to options.txt
diff: @@ -401,6 +401,7 @@ export function stringify(setting: GameSetting | Frame | any, original?: string,
  .filter((key) => key !== undefined && key !== "undefined")
  .map((key) => {
  const val = model[key];
  + if (typeof val === undefined) { return ""; }
  return typeof val !== "string" ? `${key}:${JSON.stringify(val)}` : `${key}:${val}`;
  }).join(eol);
  }
  