It's a bit arguable whether the `Subtitle` implementation supports
zero-duration events, since `getEventTimeCount` is documented as
effectively "the number of times the cues returns by `getCues(long)`
changes", and zero-duration events violate that. However, the current
`WebvttSubtitle` impl **does** produce zero-duration events, so it
seems safer to handle them gracefully here and then, as a possible
follow-up, fix the `WebvttSubtitle` impl (or remove it completely).
Issue: androidx/media#1177
#minor-release
PiperOrigin-RevId: 616095798
e9ed874e51