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>