Fixes

If ```html-differ``` will not find any ```diffs``` you will receive this:

```
gavryushin:html-differ gavryushin$ bin/html-differ test/fixtures/1.html test/fixtures/1.html

gavryushin:html-differ gavryushin$ 
```

It is not good, I think! there are no any ```diffs```, but we receive an emty string in the output!