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
}