Make processing keyframe procedure handle nulls

As reported:
http://lists.w3.org/Archives/Public/public-fx/2014OctDec/0064.html

Generated from:

commit 957350a0c2bb140a30030d70bf7a9ddc3dda5cca
Author: Brian Birtles <birtles@gmail.com>
Date:   Thu Nov 27 13:55:11 2014 +0900

    Make processing keyframe procedure handle nulls

    As reported:
    http://lists.w3.org/Archives/Public/public-fx/2014OctDec/0064.html