code: func (n *NullBool) ParseStringValue(val string) error {
  	if val == "" {
  		return nil
  	}
  
  	boolVal, err := strconv.ParseBool(val)
  	if err != nil {
  		return err
  	}
  
  	n.Value = boolVal
  	n.IsSet = true
  
  	return nil
  }
docs: // ParseStringValue is used to parse a user provided flag argument.