Let's say I have this folder structure:

_content
   home
   articles
      politics
         01.right-vs-left
         02.the-best-presidential-debates-ever
      sports
      humor
         01.american-vs-english-comedy
      design
   contact 

So, I've subdivided the articles into several folders, each being a category. However, as you can see some folders (in this example being “sports” and "design") don't have entries yet. I don't want them showing up in the nav. How do I do that automatically (only display in the nav folders with entries in them)?