Fix firstChild.nodeName error for empty slides (CSS background)

One of the members of our team wanted a background on a slide, but didn't want it to be fullscreen. So he created an empty div and used CSS to apply a tiling background to it:

```
<div class="center" data-bodyclass="unification"></div>
<style>
    .unification { background: url('http://31.media.tumblr.com/tumblr_m6dxt5mWaD1qljgz7o1_1280.gif'); }
</style>
```

This caused an error when Big attempted to check the `firstChild` property of the div (because it was empty).

`Uncaught TypeError: Cannot read property 'nodeName' of null`

So this checks to make sure there IS a firstChild before proceeding.