Fix wrong return type if empty dirty input. Again.
Fix wrong return type if empty dirty input. Again.

We MUST NOT return early here by just returning a string; it will return
the wrong type if the user wants a DOM node.

neilj/DOMPurify,Joris-van-der-Wel/DOMPurify,Joris-van-der-Wel/DOMPurify,neilj/DOMPurify,tdeekens/DOMPurify,tdeekens/DOMPurify,neilj/DOMPurify,tdeekens/DOMPurify,Joris-van-der-Wel/DOMPurify