private static String checkString(String content, String erroMessageTitle)
      throws IllegalStateException {
    checkState(
        !isNullOrEmpty(content), "require non-empty %s, but got \"%s\"", erroMessageTitle, content);
    return content;
  }
Checks a {@code content} is not null or empty and passes it through.
extensions/data-transfer/portability-data-transfer-apple/src/main/java/org/datatransferproject/datatransfer/apple/http/TokenRefresher.java