Cleans up the code and fixes two issues:
- If there are no streamheaders in the caps but we have `HEADER`
buffers, it would run `gst_buffer_list_foreach` with `self->headers`
being `NULL`.
- The code forgot to unmap the buffer if it decided to ignore it.
Fixes: 0a562a92d7ee38d8919d1b802add84d3c93b59eb
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9284>