Handle None JWT response

[TomBrien]

2022-03-09 13:08:40 DEBUG (SyncWorker_1) [cardiffwaste.cardiffwaste] Starting sorting bins
2022-03-09 13:08:40 DEBUG (SyncWorker_1) [cardiffwaste.cardiffwaste] Requesting JWT
2022-03-09 13:08:40 DEBUG (SyncWorker_1) [cardiffwaste.cardiffwaste] Completed JWT request with status code: 200
2022-03-09 13:08:40 ERROR (MainThread) [custom_components.cardiffwaste] Unexpected error fetching cardiffwaste data: Expecting value: line 1 column 1 (char 0)
Traceback (most recent call last):
File "/usr/src/homeassistant/homeassistant/helpers/update_coordinator.py", line 187, in _async_refresh
self.data = await self._async_update_data()
File "/config/custom_components/cardiffwaste/__init__.py", line 111, in _async_update_data
return await self._hass.async_add_executor_job(
File "/usr/local/lib/python3.9/concurrent/futures/thread.py", line 58, in run
result = self.fn(*self.args, **self.kwargs)
File "/usr/local/lib/python3.9/site-packages/cardiffwaste/cardiffwaste.py", line 201, in get_next_collections
response = self.get_raw_collections()
File "/usr/local/lib/python3.9/site-packages/cardiffwaste/cardiffwaste.py", line 145, in get_raw_collections
jwt = _get_token(self._user_agent)
File "/usr/local/lib/python3.9/site-packages/cardiffwaste/cardiffwaste.py", line 50, in _get_token
result = json.loads(xml.find("GetJWTResult").get_text())
File "/usr/local/lib/python3.9/json/__init__.py", line 346, in loads
return _default_decoder.decode(s)
File "/usr/local/lib/python3.9/json/decoder.py", line 337, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "/usr/local/lib/python3.9/json/decoder.py", line 355, in raw_decode
raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
2022-03-09 13:08:40 DEBUG (MainThread) [custom_components.cardiffwaste] Finished fetching cardiffwaste data in 0.274 seconds (success: False)