Usually all the requests that haven't been monetized even at the lowest possible CPM rate would display a blank, but since we don't want your users seeing blank ad spots on the site, we can set a "house ad" campaign.

The house ad is a standard image or Rich media creative, matching the respective ad unit's size, advertising publisher’s content, premium program, YouTube channel etc. The creative is hosted on your server, so you can change it whenever you like – all we need is a URL. 

By implementing this, we will deal with blanks and potentially generate additional page views.

There are also some alternative solutions, such as Amazon affiliate - if the request is not sold programmatically, an Amazon affiliate ad is displayed, and you could generate some additional revenue in the form of commission if the user interacts with it an makes a purchase.