Added into description that an empty sequence of action should result in 0 dps.

I chose 0, as technically, doing no actions should result in 0 damage, but it isn't illegal to do nothing.
But if I were to add some invalid/unexpected characters in string, a None or exception would be a good return value.

Made a function that loops and generate random test cases.

Thanks, edited the description to more precisely indicate what would happen when shooting with less than 20 energy

Made a function that loops and generate random test cases.

Made charging/shooting random lengths.