fix FutureWarning (#129)
fix FutureWarning (#129)

* fix FutureWarning

When looking at the log of HomeAssistant I came occros this:
/srv/homeassistant/lib/python3.7/site-packages/denonavr/denonavr.py:1031: FutureWarning: The behavior of this method will change in future versions.  Use specific 'len(elem)' or 'elem is not None' test instead.
This fixes that
scarface-4711/denonavr