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

src/view/form/control/AphrontFormPolicyControl.php

@@ -351,7 +351,7 @@ final class AphrontFormPolicyControl extends AphrontFormControl {
         $viewer,
         $space_phid),
       array(
-        'disabled' => $this->getDisabled(),
+        'disabled' => ($this->getDisabled() ? 'disabled' : null),
         'name' => 'spacePHID',
         'class' => 'aphront-space-select-control-knob',
       ));