I had a similar problem with my app. Up until a couple of weeks ago we were able to make requests to the Foursquare api without specifying category ids. By doing this all place categories were considered. But now if you ommit categoryId no place is returned at all. I don't know if this is a bug on the Foursquare api or just a feature change (a VERY BREAKING change).
While we don't know the real answer, try adding a "categoryId" to your request, such as:
https://api.foursquare.com/v2/venues/search?
client_id={YOUR_CLIENT_ID}
&client_secret={YOUR_CLIENT_SECRET}
&v=20130815&ll={LATITUDE},{LONGITUDE}
&limit=50
&intent=browse
&radius=50
&categoryId=4d4b7104d754a06370d81259,4d4b7105d754a06372d81259
(the indentation on the URL above is just for legibility, you should pass this URL as a single line when doing your request)