Player crashes when parsing VTT file whose last track is empty

If you try to use something like this: 
```
WEBVTT

00:00:00.000 --> 00:00:15.800 align:middle line:90%
Hello world

00:00:15.800 --> 00:00:19.800 align:middle line:90%
```

the webvtt track parser method in tracks.js will crash on line 905 cause the initial text check in the ternary condition does an explicit check against null instead of null and undefined.