Let FastCloneShallowObjectStub work with empty literal objects.

R=verwaest@chromium.org

LOG=N

BUG=417290

Review URL: https://codereview.chromium.org/655163002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24618 ce2b1a6d-e550-0410-aec6-3dcde31c8c00