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.
Maybe, but this was your idea, not mine ;) I haven't touched grunt.js: 

```javascript
      'computedStyle-valid': {
        // Generate valid JS
        src: 'lib/computedStyle.js',
        dest: validJs,
        replacements: [{
          // Replace the first function with a `var`
          from: /function/,
          to: 'var computedStyle = function'
        }]
      }
```