Nan fix

With the latest changes to specParser, if there ever was a time that you were animating a tween transition that has [number, undefined], it would return [number, NaN], preventing any animation from occurring. This adds a check to TweenTransition that only performs the interpolate if it is not equal to undefined. 

The most common occurrence of animating a [number, undefined] is when you're animating a size that is delegating one dimension of size to the parent. 

@dmvaldman