rtsp-media: Add one more case to seek avoidance

This is an extension to the previous commit. There can also be cases where the
start position is not specified, in those cases we should also avoid doing
seeking unless it's forced.

Part-of: <gstreamer/gst-rtsp-server!199>
28 jobs for !199 with backport-mr-197-into-1.18-more-avoid in 25 minutes and 55 seconds (queued for 3 seconds)
merge request