code: func builtinIsNull(args []types.Datum, _ context.Context) (d types.Datum, err error) {
  	if args[0].IsNull() {
  		d.SetInt64(1)
  	} else {
  		d.SetInt64(0)
  	}
  	return d, nil
  }
docs: // See https://dev.mysql.com/doc/refman/5.7/en/comparison-operators.html#function_isnull