Handle Peblar without customization firmware

[frenck]

Proposed Changes

There are Peblar white-labeled products around that apparently do not have customizations in their firmware.

As reported here: home-assistant/core#134022 (comment)

Unexpected error fetching Peblar Peblar version data
Traceback (most recent call last):
File "/usr/src/homeassistant/homeassistant/helpers/update_coordinator.py", line 379, in _async_refresh
self.data = await self._async_update_data()
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/src/homeassistant/homeassistant/components/peblar/coordinator.py", line 81, in handler
return await func(self, *args, **kwargs)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/src/homeassistant/homeassistant/components/peblar/coordinator.py", line 133, in _async_update_data
available=await self.peblar.available_versions(),
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/usr/local/lib/python3.13/site-packages/peblar/peblar.py", line 190, in available_versions
return PeblarVersions.from_json(result)
~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^
File "", line 8, in mashumaro_from_json
mashumaro.exceptions.MissingField: Field "customization" of type str is missing in PeblarVersions instance

This PR mitigates that, by marking this field optional. This will need a small adjustment in Home Assistant upstream, to only create the update entities when the customization firmware is present.