zero/string.go
func (s *String) UnmarshalJSON(data []byte) error {
	var err error
	var v interface{}
	if err = json.Unmarshal(data, &v); err != nil {
		return err
	}
	switch x := v.(type) {
	case string:
		s.String = x
	case map[string]interface{}:
		err = json.Unmarshal(data, &s.NullString)
	case nil:
		s.Valid = false
		return nil
	default:
		err = fmt.Errorf("json: cannot unmarshal %v into Go value of type zero.String", reflect.TypeOf(v).Name())
	}
	s.Valid = (err == nil) && (s.String != "")
	return err
}
func (s *String) UnmarshalJSON(data []byte) error {
	var err error
	var v interface{}
	if err = json.Unmarshal(data, &v); err != nil {
		return err
	}
	switch x := v.(type) {
	case string:
		s.String = x
	case map[string]interface{}:
		err = json.Unmarshal(data, &s.NullString)
	case nil:
		s.Valid = false
		return nil
	default:
		err = fmt.Errorf("json: cannot unmarshal %v into Go value of type zero.String", reflect.TypeOf(v).Name())
	}
	s.Valid = (err == nil) && (s.String != "")
	return err
}
// UnmarshalJSON implements json.Unmarshaler.
// It supports string and null input. Blank string input produces a null String.
// It also supports unmarshalling a sql.NullString.