R1:b9fd5262509d

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…