SDK / exoplayer

`subtitle` is only guaranteed to be non-null if
`nextSubtitleEventIndex != C.INDEX_UNSET`. The null check added in
https://github.com/google/ExoPlayer/commit/0efec5f6c12a5d583f24c122fbcbc1b1eebbabc3
was too early.

Issue: #8017
PiperOrigin-RevId: 334777742