converting NULL to string is unsupported

[vizvasrj]

how to convert null to string .?

here token is null and I want it to convert it to""empty string

func find_a_user_with_username(db *sql.DB, username string) (models.Users, error) {
	if username == "" {
		return models.Users{}, myerror.New("invalid username is none")
	}

	var user models.Users

	err := db.QueryRow(`
	SELECT id, username, password_hash, 
		email, token, refresh_token, 
		created_at, updated_at
	FROM users WHERE username = $1
	`, username).Scan(
		&user.Id, &user.Username, &user.PasswordHash,
		&user.Email, &user.Token, &user.RefreshToken,
		&user.CreatedAt, &user.UpdatedAt,
	)
	if err != nil {
		if err == sql.ErrNoRows {
			return user, myerror.New("No matching rows found.")
		} else {
			return user, myerror.WrapError(err, "Error retriving row")
		}
	}
	return user, nil
}