Improve an awkward initialization behavior in Javelin in Firefox

Summary: See comment; see IRC. This didn't cause anything bad, but ended up with document.body.id == "null" in Firefox, which is a bit weird.

Test Plan: Loaded page, put document.body.null into the console, got consistent results.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7288