Web Animations: Use isNull() and nullValue() in KeyframeAnimationEffect

Clean up only, no change in behavior.

R=dstockwell

Review URL: https://codereview.chromium.org/98003002

git-svn-id: svn://svn.chromium.org/blink/trunk@162941 bbb929c8-8fbe-4397-9dbb-9b2b20218538