How do I send user object as nil (null in json) in Golang?

I am writing some authentication using Golang, and I have the following code that populates the UserResponse if UserID is not nil, but when it IS nil, the sent UserResponse looks like this:

{
    "user": {
        "id": 0,
        "email": ""
    }
}

My code is

func (app *Application) init(w http.ResponseWriter, r *http.Request) {
	userID := r.Context().Value("userID")

	type UserResponse struct {
		ID int `json:"id"`
		Email string `json:"email"`
	}

	var user UserResponse

	if userID != nil {
		query := "select id, email from users where id = ?"
		row := app.Db.QueryRow(query, userID)
		err := row.Scan(&user.ID, &user.Email)
		if err != nil {
			fmt.Println(err.Error())
		}
		fmt.Println(user)
	}

	app.writeJSON(w, http.StatusOK, user, "user")
}

it should be like this

{
    userID := r.Context().Value("userID")

	type User struct {
		ID    int    `json:"id"`
		Email string `json:"email"`
	}

	type Response struct {
		User *User `json:"user"`
	}

	var resp Response

	if userID != nil {
		user = &User{}
		query := "select id, email from users where id = ?"
		row := app.Db.QueryRow(query, userID)
		err := row.Scan(&user.ID, &user.Email)
		if err != nil {
			WriteJSON(w, http.StatusInternalServerError, resp)
			return 
		}
		resp.User = user
	}

	WriteJSON(w, http.StatusOK, resp)
}

I found a solution. First I created an interface{} and populated it either nil or with a user depending on whether UserID is present. However, I wanted a better cleaner solution and came up with this.

	var user *UserResponse

	if userID != nil {
		user = &UserResponse{}
		query := "select id, email from users where id = ?"
		row := app.Db.QueryRow(query, userID)
		err := row.Scan(&user.ID, &user.Email)
		if err != nil {
			fmt.Println(err.Error())
		}
		fmt.Println(user)
	}