Fix WMF Video not playing if no soundcard is available

Do not add the AudioEndpoint to the topology if no Audio output device
is available. Fixes video not playing if you deactivate your soundcard
or have no headphones/speakers plugged in.

Change-Id: I9fc2486198a299b3e75af648f69475270968c6f7
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>