Correct support for SVGAnimatedStrings

This corrects the incomplete fix for `SVGAnimatedString`s from https://github.com/walmartlabs/costanza/pull/13
`this.className.baseVal != null ?`... is preferred as it's a bit shorter when minimized.

Is there anyway that this case can be added to the tests? That's the surefire way to ensure that this does not regress in the future.