Permit some config_entry options to be unset without throwing

[aldahick]

Not sure what changed, but when I updated my installation to the latest commit, it started throwing:

2023-05-02 08:09:00.843 ERROR (MainThread) [homeassistant.components.light] Error while setting up cync_lights platform for light
Traceback (most recent call last):
  File "/usr/src/homeassistant/homeassistant/helpers/entity_platform.py", line 304, in _async_setup_platform
    await asyncio.shield(task)
  File "/config/custom_components/cync_lights/light.py", line 25, in async_setup_entry
    if not hub.cync_rooms[room]._update_callback and (room in config_entry.options["rooms"] or room in config_entry.options["subgroups"]):
KeyError: 'subgroups'

It seems safe to assume that an unset value is the same as an empty value, so this PR does that.