Empty FSM states do not produce expected behavior

[whitequark]

E.g. this should produce an FSM that goes to the B state and stays there:

self.submodules.fsm = FSM(reset_state="A")
self.fsm.act("A", NextState("B"))
self.fsm.act("B")

but it does something else.

[whitequark]

Triage: fixed in nMigen.