handling null objects from getComputedStyle

Sometimes Firefox behaves weirdly and returns null for window.getComputedStyle.
Case reproduce (look into console in FF and some other browser): http://jsfiddle.net/bvk1bhmo/
Bug report at Mozilla: https://bugzilla.mozilla.org/show_bug.cgi?id=548397

Then computedStyle tries to get on a property of null object and it blows up, stopping further execution. A simple try/catch and returning an empty string would at least prevent dependent scripts execution halt.
A global try/catch can cause a lot of undesired issues (e.g. hiding errors outside of the iframe case -- referring to an undeclared variable). I would prefer if we check against `null` before the property access instead:

```
// Get the computed style
var style = getComputedStyle ? getComputedStyle(el) : el.currentStyle;

// If we have a style, return its property
// DEV: This covers FF returning `null` for when this is used in an iframe
if (style) {
  var camelCaseName = prop.replace...
  return style[camelCasename];
// Otherwise, return nothing
} else {
  return void 0;
}
```