Well, the wording states that this is 'extra' damage, which would imply that it's only applied if there is damage in the first place, otherwise it wouldn't be extra to anything.
But then again, that's a pretty obtuse ruling based on one word, so I wouldn't take that as gospel.
I think this is really a case best served by RAI. A power that does no damage seems obviously intended to do no damage, period. Critting, therefore, means nothing.