The `SefSlowMotionVideoSampleTransformer` drops frames by
setting `buffer.data = null` and updates the timestamps of the
frames it keeps. However, these buffers with dropped frames
are still used in the pipeline as they may contain other useful
information, specifically whether the end of the input has been
reached. So, the timestamps should also be updated for the buffers
where frames are dropped.
PiperOrigin-RevId: 410797678
