code: func (b *builtinNullTimeDiffSig) evalDuration(row chunk.Row) (d types.Duration, isNull bool, err error) {
  	return d, true, nil
  }
docs: // evalDuration evals a builtinNullTimeDiffSig.
  // See https://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_timediff