If you don't have anything, you don't have anything. Less is sometimes more if you feel like it will sound awkward and forced. Take a little time and really consider if there's anything you want to say. This is their application, so I would expect it will be given closer attention. It's your last shot to say anything directly and catered to the school before they decide on you. But I don't think it's a big deal if you leave it blank if you've got nothing to say.