findByArg should return an iterable, not NoneType

[simon-ess]

In https://github.com/ChannelFinder/pyCFClient/blob/9689d5bf/channelfinder/ChannelFinderClient.py#L334 we have that findByArgs returns None if the client gets a 404. However, in cfstore.py from recsync, we have

        for ch in client.findByArgs(prepareFindArgs(conf, [('~name', eachSearchString)])):
            existingChannels[ch["name"]] = ch

which results in repeated errors such as

2023-08-24T11:47:19+0000 [-] ERROR:recceiver.cfstore CF_COMMIT FAILURE: [Failure instance: Traceback: <class 'TypeError'>: 'NoneType' object is not iterable
	/usr/local/lib/python3.10/threading.py:1016:_bootstrap_inner
	/usr/local/lib/python3.10/threading.py:953:run
	/home/recceiver/venv/lib/python3.10/site-packages/twisted/_threads/_threadworker.py:47:work
	/home/recceiver/venv/lib/python3.10/site-packages/twisted/_threads/_team.py:182:doWork
	--- <exception caught here> ---
	/home/recceiver/venv/lib/python3.10/site-packages/twisted/python/threadpool.py:244:inContext
	/home/recceiver/venv/lib/python3.10/site-packages/twisted/python/threadpool.py:260:<lambda>
	/home/recceiver/venv/lib/python3.10/site-packages/twisted/python/context.py:117:callWithContext
	/home/recceiver/venv/lib/python3.10/site-packages/twisted/python/context.py:82:callWithContext
	/home/recceiver/venv/lib/python3.10/site-packages/recceiver/cfstore.py:275:_commitWithThread
	/home/recceiver/venv/lib/python3.10/site-packages/recceiver/cfstore.py:558:poll
	/home/recceiver/venv/lib/python3.10/site-packages/recceiver/cfstore.py:464:__updateCF__
	]

in the log. It would be better to return an empty list or some other empty iterable, or to even raise an exception to avoid this junk log data.

[simon-ess]

Note that this could possible also be fixed on the recsync side; however, it seems generally better to return consistent data types where possible.