device: fix comparison in _has_classesv()
device: fix comparison in _has_classesv()

We're comparing a pointer type with '\0' here, which
probably isn't right, and the loop condition made sure
that classes[0] is != NULL already, so it's pointless.
Was probaby meant to check if the string pointed to is
not empty, so make it do that instead.

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