Empty VTT file not supported in Shaka Player

[kqyang]

Have you read the FAQ and checked for duplicate open issues?:

Yes.

What version of Shaka Player are you using?:

Latest demo.

Can you reproduce the issue with our latest release version?:

Yes.

Can you reproduce the issue with the latest code from master?:

Are you using the demo app or your own custom app?:

Demo app.

If custom app, can you reproduce the issue using our demo app?:

What browser and OS are you using?:

Chrome on Linux.

What are the manifest and license server URIs?:

https://storage.googleapis.com/wvtemp/kqyang/vtt0/bourne_1000k.m3u8

What did you do?

Click "Load" to play the media.

What did you expect to happen?

Content plays.

What actually happened?

Shaka Error TEXT.UNABLE_TO_EXTRACT_CUE_START_TIME (TypeError: Cannot read property 'startTime' of undefined)

The VTT files referenced in the text playlists are empty with just one line:

WEBVTT

The playlist plays in Safari and hls.js. Also, The file is compliant to WEBVTT spec here: https://www.w3.org/TR/webvtt1/#file-structure.

Let me know if I missed anything.

[joeyparrish]

Replaced by #1563, as this is part of a broader issue we have with HLS text.