whipclientsink: pushing to twitch.tv's WHIP endpoint is a noop

Pushing from GStreamer to twitch.tv via WHIP seems to run fine client-side but nothing is registering on the twitch.tv side.

To reproduce:

# I also tried with `do-fec=false` but same result
gst-launch-1.0 -v whipclientsink name=whip signaller::whip-endpoint=https://g.webrtc.live-video.net:4443/v2/offer signaller::auth-token=TWITCH_KEY videotestsrc ! videoconvert ! x264enc speed-preset=veryfast tune=zerolatency ! h264parse ! whip. audiotestsrc ! audioconvert ! opusenc ! whip.

See also: https://www.reddit.com/r/WebRTC/comments/12nc5md/twitchtv_now_supports_webrtc_ingestion_via_whip/

By contrast OBS works fine and https://github.com/ggarber/whip-go works reasonably well as well.

N.B. my actual use case would be using an rtspsrc as input but that's a bit overloaded for the scope of this issue, hence starting with testsrcs.

This is with GStreamer 1.24.0 for all modules.