According to the HLS+WebVTT spec, if there is no text in a segment
a webvtt file with no cues can be added to the manifest. By outputting
empty segments it allows the accumulated duration in the Master Playlist
to better represent the duration of the text stream.
Spec Reference: //tools.ietf.org/html/draft-pantos-http-live-streaming-23
Section 3.5. WebVTT
Bug: #205
Change-Id: I5de01200fd9fa99c57949c773e8ee926b0f6ba8a