Fix linting of html files that don't contain any JavaScript

Right now if you try to extract JavaScript from an HTML file that does not contain any, the plugin will try to lint the original HTML instead. I've changed the corresponding check so that for any file that went through the extractor, the extracted string will be used for linting even if it is empty. This is useful if you want to lint a bunch of html files, some of which have JavaScript in them and some of which do not.
Maybe it would be a bit more readable is you used `_.isString()