Fix handling HomeKit events when the char is in error state

[bdraco]

Proposed change

Fixes the below which was introduced in #97869 by moving the check deeper down so it shouldn't get there if the char has an error status

I didn't see this when testing the original PR because I didn't have any chars returning -70402

2023-08-05 18:37:50.146 DEBUG (MainThread) [homeassistant.components.homekit_controller.device_trigger] Event (11158856,67) {'status': -70402, 'description': 'Unable to communicate with requested service, e.g. the power to the accessory was turned off.'}
2023-08-05 18:37:50.146 ERROR (MainThread) [homeassistant.components.homekit_controller.connection] Unexpected exception from <bound method HKDevice.async_update of <homeassistant.components.homekit_controller.connection.HKDevice object at 0x7fb3787fa690>>
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/helpers/debounce.py", line 116, in _handle_timer_finish
    await task
  File "/usr/src/homeassistant/homeassistant/components/homekit_controller/connection.py", line 786, in async_update
    self.process_new_events(new_values_dict)
  File "/usr/src/homeassistant/homeassistant/components/homekit_controller/connection.py", line 797, in process_new_events
    async_fire_triggers(self, new_values_dict)
  File "/usr/src/homeassistant/homeassistant/components/homekit_controller/device_trigger.py", line 271, in async_fire_triggers
    if ev["value"] is None:
       ~~^^^^^^^^^
KeyError: 'value'

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[home-assistant]

Hey there @Jc2k, mind taking a look at this pull request as it has been labeled with an integration (homekit_controller) you are listed as a code owner for? Thanks!

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[bdraco]

pushed to the system with the device that has the unavailable char. all good now