controller: Do not unset uninitiallized GValue
controller: Do not unset uninitiallized GValue

In case the property was not interpollable we might never initialize
the GValue, we should thus never unset it.

StreamUtils/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,centricular/gstreamer