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