Fix incorrect rendering of "Spaces" dropdown control when configuring EditEngine defaults
Summary:
Ref T10004. This could sometimes pass false, which counts as disabled.
Instead, pass null explicitly.
Test Plan: Edited default space on an EditEngine form.
Reviewers: chad
Reviewed By: chad
Maniphest Tasks: T10004
Differential Revision: https://secure.phabricator.com/D14858