WebVTT cues without payload may not carry meaningful information, but it
is allowed by WebVTT specification [1]. It could also be useful
sometimes, e.g. to signal the time progression in live case.
Fixes#433.
[1] https://www.w3.org/TR/webvtt1/#types-of-webvtt-cue-payload
Change-Id: I9e31f4a3789cbdafb7667b64f4019834190ecfc0