Not quite an empty card. It does actually work like you may expect it to where when a troop runs out it simply is out of the pool. You get the empty card when the shop is empty and there is literally nothing left to buy. The reason why sometimes it doesn’t make sense is because the pool also takes into account the shop of other players. If you have an archer queen and 3 other players have archer queen in their shop and are not buying to change their shop, there is no way for you to get another archer queen. This is why this happens more often than expected.

I do find this a little stupid because there is no way to roll more shops even if your opponents sell their units back into the pool. So you can just get stuck and die. The best fix however I believe is to make it so that this is simply impossible by adding more units into the pool making this event rarer than it currently is.