netmeg - 2:36 pm on Aug 2, 2012 (gmt 0)
I have no idea if this is really an issue or not. I've never ever noticed a time when there were no ads in an ad unit. Isn't that what it means? That an ad wasn't available?
I guess if thought it might be a problem, I might set my own ad or graphic as the fill for an ad unit, and then after a few days look at my logs to see how many times the graphic displayed.