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.
Adding the defensive `undef` isn't necessary. It doesn't help with optimizers and if someone has overriden `undefined`, then they have larger problems than this lib.