code: protected static void failWithMessage(@Nullable String customErrorMessage, Throwable realCause) {
      if (customErrorMessage != null) {
        fail(customErrorMessage, realCause);
      }
    }
docs: Throws an {@code AssertionError} only if the given custom message is not {@code null}.
  
  @param customErrorMessage the custom error message.
  @param realCause          cause of the error.
  @throws AssertionError only if the custom error message is not {@code null}.
  @since 1.2