Promises: make null a legal argument for .then

R=svenpanne@chromium.org
BUG=

Review URL: https://codereview.chromium.org/203453002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20037 ce2b1a6d-e550-0410-aec6-3dcde31c8c00