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

And not a NULL Iterator, so it is consistent with the way it usually
works and avoid user to need a different code paths to handle that.

rawoul/gst-plugins-good,GrokImageCompression/gst-plugins-good,sh0/gst-plugins-good,chamois94/gst-plugins-good,GrokImageCompression/gst-plugins-good,kittee/gst-plugins-good,Kurento/gst-plugins-good,hizukiayaka/gst-plugins-good,veo-labs/gst-plugins-good,BigBrother-International/gst-plugins-good,loshca/gst-plugins-good,froggatt/gst-plugins-good-m,GrokImageCompression/gst-plugins-good,loshca/gst-plugins-good,stfl/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,sh0/gst-plugins-good,StreamUtils/gst-plugins-good,vatavuserban/gst-plugins-good,froggatt/gst-plugins-good-m,ijsf/OpenWebRTC-gst-plugins-good,loshca/gst-plugins-good,pexip/gst-plugins-good,hizukiayaka/gst-plugins-good,veo-labs/gst-plugins-good,kittee/gst-plugins-good,GStreamer/gst-plugins-good,pexip/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,Kurento/gst-plugins-good,GStreamer/gst-plugins-good,ikonst/gst-plugins-good,hizukiayaka/gst-plugins-good,stfl/gst-plugins-good,Kurento/gst-plugins-good,chamois94/gst-plugins-good,veo-labs/gst-plugins-good,vatavuserban/gst-plugins-good,pexip/gst-plugins-good,pexip/gst-plugins-good,froggatt/gst-plugins-good-m,ikonst/gst-plugins-good,kittee/gst-plugins-good,GStreamer/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,shelsonjava/gst-plugins-good,sebras/gst-plugins-good,stfl/gst-plugins-good,BigBrother-International/gst-plugins-good,rawoul/gst-plugins-good,shelsonjava/gst-plugins-good,sh0/gst-plugins-good,sebras/gst-plugins-good,veo-labs/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,sebras/gst-plugins-good,BigBrother-International/gst-plugins-good,StreamUtils/gst-plugins-good,shelsonjava/gst-plugins-good,BigBrother-International/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,froggatt/gst-plugins-good-m,GStreamer/gst-plugins-good,ikonst/gst-plugins-good,Kurento/gst-plugins-good,chamois94/gst-plugins-good,shelsonjava/gst-plugins-good,vatavuserban/gst-plugins-good,kittee/gst-plugins-good,GrokImageCompression/gst-plugins-good,rawoul/gst-plugins-good,ikonst/gst-plugins-good,Kurento/gst-plugins-good,sh0/gst-plugins-good,StreamUtils/gst-plugins-good,vatavuserban/gst-plugins-good,hizukiayaka/gst-plugins-good,ijsf/OpenWebRTC-gst-plugins-good,pexip/gst-plugins-good,rawoul/gst-plugins-good,chamois94/gst-plugins-good,StreamUtils/gst-plugins-good,sebras/gst-plugins-good,freedesktop-unofficial-mirror/gstreamer__gst-plugins-good,stfl/gst-plugins-good,loshca/gst-plugins-good