Fix ElementTree warnings

[jjlawren]

As seen in #515 there are warnings similar to this printed when a plain boolean check is made on an ElementTree object:

/srv/homeassistant-3.7/lib/python3.7/site-packages/plexapi/client.py:72: FutureWarning: The behavior of this method will change in future versions.  Use specific 'len(elem)' or 'elem is not None' test instead.
  if not any([data, initpath, baseurl, token]):

This PR makes more explicit checks against None to avoid these warnings.