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%"`

So by changing the order here I think it might kill the FF bug for
`'background-position'`