Crash when calling getCueAsHTML() on a TextTrackCue with empty text.

Even if the TextTrackCue text is empty, a document fragment should still be returned as a result value when calling getCueAsHTML(). Currently, the call crashes the browser, when the cue text is "".

TEST=Added relevant test case.
R=acolwell,jchaffraix
BUG=261653

Review URL: https://chromiumcodereview.appspot.com/22304002

git-svn-id: svn://svn.chromium.org/blink/trunk@155890 bbb929c8-8fbe-4397-9dbb-9b2b20218538