Antecipated backgroundPosition check

When element has no background-position values or after this happens in the testing:
`element.setStyle('background-position', null);` 

Then inside getStyle:

    var result = this.style[property];       // gives ""
    if (!result || property == 'zIndex'){    //gives true from !result

So it goes inside the `if` and calls `result = this.getComputedStyle(property);` 
which is in FF `"0% 0%"` and therefore the Specs error `Expected '0% 0%' to equal '0px 0px'.`

So by changing the order here it returns `'0px 0px'` and makes FF happy and the error/bug for
`'background-position'` goes away