so what happens if a ps 3 pilot with deadman's switch goes over an asteroid, dies, assigns a damage card to null, making his ps go to 0. does he not activate, since ps 0 has already gone?

The most relevant text from the rules reference I am aware of states, "The Activation Phase is the third phase of a round. During this phase, each ship activates, one at a time, starting with the ship with the lowest initiative and continuing in ascending order."

Since Null would be the ship with the lowest initiative at that point, he would then activate next. This would be the same behavior as Epsilon Ace from First Edition as answered in this rules question.