queue2: Don't report 0% unless empty
queue2: Don't report 0% unless empty

When preparing a buffering message, don't report 0% if there
is any bytes left in the queue at all. We still have something
to push, so don't tell the app to start buffering - maybe
we'll get more data before actually running dry.

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