go get github.com/emvi/null

package main
import (
    "encoding/json"
    "database/sql"
    "fmt"
    "github.com/emvi/null"
)
type NullableString struct {
    Value null.String `json:"value"`
}
func main() {
    str := NullableString{null.NewString("nullable string", true)}
    // or long version: str := NullableString{null.String{sql.NullString{String: "nullable string", Valid: true}}}
    
    data, _ := json.Marshal(str)
    fmt.Println(string(data)) // -> {"value": "nullable"}
    str.SetNil() // use str.SetValid("value") to set a value again
    data, _ = json.Marshal(str)
    fmt.Println(string(data)) // -> {"value": null}
}