The Impact will fire without a pellet in the chamber. Don't do it. Most of the time, no issue, but once in a while, you can blow an O-Ring, and possibly damage the valve seat.
I did fire mine without a pellet, or barrel for that matter. I had the barrel off and didn't realize It was cocked (stupid time) and bumped the trigger while I was changing the barrel to a new STX. BANG! and immediately destroyed the O-Rings around the regulator, and destroyed the valve seat.
If you have any questions, call FX USA in North Carolina and ask them directly. They'll give you absolute correct answer as to whether it damages the gun or not. 866-639-0772. You can either have the person answering the phone ask the tech while you wait for the answer (they did that for me), or set an appointment to have the tech call you back (and you might even get Ernest Rowe). They did that for me on another call.
It's a phenomenal gun, you are incredibly lucky to have one.