Under Robert's, the unopposed candidate wins the office by acclamation.
You can add to your bylaws a provision that would allow an up-or-down vote on an unopposed candidate, but it's questionable whether that would be wise. For one thing, it raises the question of what happens if "no" wins. Your bylaw would need to cover that.
One might also wonder why there is not a "no" (or "none of the above") option when there are two or more candidates. Why should an unopposed candidate have to run against "no," when opposed candidates have to run against only each other?