Remove blocks if there are no linked assets

Before a link was created, causing a 404. (yeoman/generator-webapp#242)

```html
<!-- build:css styles/vendor.css -->
<!-- endbuild -->
```

Btw, I was really trying to avoid doing any code style changes, but my Vim replaced all tabs with spaces when I saved the file (it was reading from .editorconfig), so I decided to fix JSHint problems while I was at it.