`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
