Manual rollback of https://github.com/google/ExoPlayer/commit/b3f485d7d9c08e39574b72a949166ee4834c3b24
It's technically possible to output a seekable SeekMap with unknown duration. This can occur if the media defines seek points but doesn't define either the overall duration or the duration of the media from the last seek point to the end.
PiperOrigin-RevId: 285769121
