在golang有没有类似于sql.NullString的sql.NullJson？

import (
    "database/sql"
    "encoding/json"
    "fmt"
)

type NullJson struct {
    sql.NullString
}

type MyStruct struct {
    Field NullJson `json:"field"`
}

func main() {
    jsonStr := `{"field": {"key": "value"}}`

    var data MyStruct
    err := json.Unmarshal([]byte(jsonStr), &data)
    if err != nil {
        fmt.Println("Error:", err)
        return
    }

    if data.Field.Valid {
        fmt.Println("Field:", data.Field.String)
    } else {
        fmt.Println("Field is NULL")
    }

    // Modify the JSON field
    data.Field.String = `{"new_key": "new_value"}`
    data.Field.Valid = true

    newJsonStr, err := json.Marshal(data)
    if err != nil {
        fmt.Println("Error:", err)
        return
    }

    fmt.Println("New JSON:", string(newJsonStr))
}