This fixes two things in one go:
1. In order to indicate 'end of a cue' **without** relying on
`CuesWithTiming.durationUs`, `SubripParser` should have been emitting
an empty `List<Cue>` instead of the current behaviour of a
single-item list with `cue.text=""`.
2. There's no need for the empty cue (or cue list), we can use
`durationUs` to indicate the end of each list of cues.
There's no real need to ever have a `Cue` with `text=""`, so also
deprecate `Cue.EMPTY`.
PiperOrigin-RevId: 548938874
0fa66534fe