code: func (b *builtinSubTimeStringNullSig) evalString(row chunk.Row) (string, bool, error) {
  	return "", true, nil
  }
docs: // evalString evals a builtinSubTimeStringNullSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_subtime