Avoid null transform error in polyfill

The Web Animations Polyfill no longer reports an error when the SVG
transforms for the motion path complete, as we use '' instead of null as
the default transform value.

Similarly, we avoid reporting errors during motion path animations when additive="sum".
animateMotion-iterationComposite.html exercises this case.
Merge pull request #13 from ewilligers/avoid-null-transform-error

Avoid null transform error in polyfill