Whenever a light turns off, I see this in my HomeAssisstant logs:

    Error doing job: Task exception was never retrieved (None)
    Traceback (most recent call last):
    File "/config/custom_components/circadian_lighting/switch.py", line 384,
    in _light_state_changed
    assert new_state and new_state.state == "on"
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    AssertionError`

This is caused by an `assert` in the `_light_state_changed` function.

I've refactored `_light_state_change` to remove the `assert` while while
keeping the existing function behavior.

When claytonjn#258 replaced `async_track_state_change` with
`async_track_state_change_event` due to upstream changes, there was a
change in the upstream function signature.

Previously, `async_track_state_change` took a `state` argument, so
events were only triggered when a light's state was `on`, so the
assertion in `_light_state_changed` was probably unneeded still, but
fine.

Now, `async_track_state_change_event` sends every event to the
`_light_state_changed` callback. So when the light state changes to
"off" or "unavailable", this assertion throws an exception.

My refactor determines if the light has turned on and triggers a forced
update if so. This is the same logic as before my change, but without an
assertion.

Fixes: claytonjn#262