#2527 exposes other issue causing constant rerequesting of valid TTML segments

[davemevans]

It is totally valid to have a segment containing a valid TTML document which contains nothing to display and this is required in order not to have gaps in the timeline.

Unfortunately, #2527 in conjunction with an existing dubious piece of code (https://github.com/Dash-Industry-Forum/dash.js/blob/development/src/streaming/utils/TTMLParser.js#L156) causes the player to continually re-request and discard perfectly valid TTML.

The TTML parser and text source buffer need to be more intelligent - it should be possible to determine the difference between valid empty documents and those which are erroneous and handle the situation accordingly.

I'll submit a patch.