Fix a fatal on user email settings when `account.editable` is disabled
Summary:
If `account.editable` is set to false, we try to add a `null` button and fatal:
> Argument 1 passed to PHUIHeaderView::addActionLink() must be an instance of PHUIButtonView, null given, called in /srv/phabricator/phabricator/src/applications/settings/panel/PhabricatorSettingsPanel.php on line 290
Instead, don't try to render `null` as a button.
Test Plan:
- Configured `account.editable` false.
- Viewed email address settings.
- Before: fatal.
- After: page works, no button is provided.
Reviewers: amckinley
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D18677