Web Animations CSS: Fix release crash when animating 'none' images

StyleImages on render styles are allowed to be null meaning the CSS
keyword none was used as the value.
This patch handles the null case and properly animates it as the none
keyword.

BUG=257591

Review URL: https://codereview.chromium.org/46883006

git-svn-id: svn://svn.chromium.org/blink/trunk@160899 bbb929c8-8fbe-4397-9dbb-9b2b20218538