If there is only unknown stream-type streams in the current collection don't post an error straight away. This fixes a problem with RTSP cameras and legacy upstream collection building, if the first stream that rtspsrc outputs is the ONVIF metadata track. That happens often on bandwidth-constrained camera inputs, as the video and audio will naturally take longer to arrive. Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9433>