iOS dev @ Foursquare here. While we don't really document support for launching the app without any arguments, we designed the URL handling system to essentially do nothing in response to invalid or no arguments.
Unfortunately, you've hit a regression in our app that causes a modal to open anyway, so this isn't going to work for the time being. It'll soon be fixed in a future version of the app.
If there's something more specific that you're trying to accomplish, feel free to reply to this answer. Thanks!