ytdl_hook: use tbr for all tracks if vbr/abr not available

vbr and abr are the video and audio bitrates. Sometimes there is a weird
mix of any of them available, but in these cases, it's not good to fall
back to tbr if a specific track has no vbr/abr.

For example, the alphabetic site provides tbr only for the muxed
fallback stream, but using tbr would make the primitive mpv hls_bitrate
selection pick the compatibility stream for audio, because it appears to
have a higher bitrate than the other audio-only streams (because the
bitrate includes video). So we must not use tbr in this case.

On the other hand, formats coming from youtube-dl HLS master playlist
use will only have tbr set.

So as a heuristic, use the tbr only if it's the only bitrate available
in any track entry.

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