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