<track>'s track URL concept is used incorrectly

[annevk]

It seems like it can be the empty string, but callers don't account for that and assume it's always a URL (which is also somewhat wrong, it's a string).

cc @whatwg/media

[domenic]

I also suspect that in implementations this is calculated dynamically, not just when src changes. (The difference would be whether it respects base URL changes properly.)