rtpjitterbuffer: Use an empty iterator in iterate_internal_link when no links
rtpjitterbuffer: Use an empty iterator in iterate_internal_link when no links

We used to setup an iterator with 1 GValue set with a NULL object
pointer which is not the normal way to do that. Instead we should make
sure that the first call to gst_iterator_next returns GST_ITERATOR_DONE.

wkatsak/gst-plugins-good,jpakkane/gstreamer-plugins-good,vatavuserban/gst-plugins-good,ndufresne/gst-plugins-good,jpakkane/gstreamer-plugins-good,jpakkane/gstreamer-plugins-good,ndufresne/gst-plugins-good,wkatsak/gst-plugins-good,wkatsak/gst-plugins-good,vatavuserban/gst-plugins-good,jpakkane/gstreamer-plugins-good,wkatsak/gst-plugins-good,ndufresne/gst-plugins-good,ndufresne/gst-plugins-good,vatavuserban/gst-plugins-good,vatavuserban/gst-plugins-good