This avoids an invalid D-Bus call during interface initialization.
The wpa_state change can happen before the D-Bus interface is set up,
so we must be preparted to handle this early event signal. In theory,
it should be possible to reorder initialization code to make sure
D-Bus signals are ready, but that would likely require quite a bit of
code restructuring, so it looks like a safer option for now is to just
skip the early event.