message: fix(webref): return undefined when ENOENT
diff: @@ -26,11 +26,11 @@ function tryGetCss(title: string) {
  let path = new URL(`${title}.json`, webrefCssDir);
  const data = tryRequire(fileURLToPath(path));
  if (!data) {
  - return "";
  + return;
  }
  const properties = Object.keys(data.properties);
  if (!properties.length) {
  - return "";
  + return;
  }
  return generateWebIdlFromCssProperties(properties);
  }
  