如何编组sql.NullString以便将输出展平以仅给出go中的值？

这是代码，

package mainimport (    "database/sql"    "encoding/json"    "fmt"    "log")//Company detailstype Company struct {    ID  int        `json:"id"`    Abn NullString `json:"abn"`}//NullString is a wrapper around sql.NullStringtype NullString sql.NullString//MarshalJSON method is called by json.Marshal,//whenever it is of type NullStringfunc (x *NullString) MarshalJSON() ([]byte, error) {    if !x.Valid {        return []byte("null"), nil    }    return json.Marshal(x.String)}func main() {    company := &Company{}    company.ID = 68    //create new NullString value    nStr := sql.NullString{String: "hello", Valid: true}    //cast it    company.Abn = NullString(nStr)    result, err := json.Marshal(company)    if err != nil {        log.Println(err)    }    fmt.Println(string(result))}

这是博客文章，详细解释了它。