acqEngine: Fix bug causing spurious setting of Z position

In a time-channel acquisition where the channels are hardware-triggered
and the interval is large enough that separate time points are run as
separate bursts, the Z position was being set between each time point
for no good reason.

The bug was that the last-stage-positions field of the state map was
being set to null after each burst lacking a hardware-triggered Z stack.

Regression test added.

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14503 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd