It is very unfortunate that it was empty, but lucky in another way ! Imagine if it was full off fresh eggs and you had killed them all :shock:
It is always best to be patient and wait beyond the normal incubation time just incase it hatches.
It really isn't a good idea to cut an ootheca at all, what it is the point when you'll find out in the end anyway ?
If you have already waited then I apologize :wink: