Check for response.html before using indexOf on it.

This is a sanity check and should be there in the off-chance that `response.html` is `null` (yes I have seen it happen). Bottom line, if there is no `response.html`, then it's obvious that media player JS will not be needed.
Merge pull request #1443 from jtsternberg/patch-1

Check for response.html before using indexOf on it.