This is a non story. The AT4 has one shot, period. This one was empty, which means it is only as deadly as any other length of pipe when used as a club(probably not as effective actually). There are no charges because the person didn’t break the law.
Bottom line, WRT "charges", based on what has been observed. One can only hope further investigating/informing is being done based on the questions naturally raised under the circumstances.
Sadly, I won't be taking any bets on whether or not that is being done.