docs: private static Optional<String> sanitizeRedirectUrl(@Nullable String url) {
      if (Strings.isNullOrEmpty(url)) {
        return empty();
      }
      if (url.startsWith("//") || url.startsWith("/\\")) {
        return empty();
      }
      if (!url.startsWith("/")) {
        return empty();
      }
      return Optional.of(url);
    }