I imagine it varies by the pump. I have no experience with the medtronic, but I posted about the OmniPod’s “reserve tank” here:
the-omnipods-reserve-tank
(On this 
thread, @Gary said the medtronic reserve is about 10 units, and @Thomas said that for Animas and Tandem, zero means zero)
Just like in a car, when your fuel gauge says you are empty, you may not be exactly empty yet. But you know pretty soon you will be stuck on the side of the road. So unless you know exactly how much you have, there is no reason to push it too much.
Is there nothing on the pump that beeps to let you know it is actually empty? On the pods, it will sound an alarm when it actually gets to zero. I just always assumed that is something that all pumps would do.
It might be useful for you, when you first see a zero, to take the reservoir out and extract insulin with a syringe and see what is left in there. Do that a few times so you know how much you have left when you see zero.