Some streams in the wild have empty segments at the beginning and
hlsdemux2 use to stall forever while we can handle it by processing
the following ones. The gap handling mechanism in adaptivedemux2
works properly and pushes the required gaps.
Update validate medias so the hls stream is present.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6034>
