I have never had it show no ads at all - generally public service (charity) ads. However, they now have an option - Alternative Ad Code - where you can have it display the banner of your choice in place of public service ads when there are no relevant ads to display on your page. Log in to your Adsense account and check that out - it may be the perfect solution for you.
If you have problems with the code, see - Henrik was a great help when I was adding the Alternative Ad Code to my pages
