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