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

src/applications/settings/panel/PhabricatorEmailAddressesSettingsPanel.php

@@ -138,9 +138,9 @@ final class PhabricatorEmailAddressesSettingsPanel
         $editable,
       ));
 
-    $button = null;
+    $buttons = array();
     if ($editable) {
-      $button = id(new PHUIButtonView())
+      $buttons[] = id(new PHUIButtonView())
         ->setTag('a')
         ->setIcon('fa-plus')
         ->setText(pht('Add New Address'))
@@ -149,7 +149,7 @@ final class PhabricatorEmailAddressesSettingsPanel
         ->setColor(PHUIButtonView::GREY);
     }
 
-    return $this->newBox(pht('Email Addresses'), $table, array($button));
+    return $this->newBox(pht('Email Addresses'), $table, $buttons);
   }
 
   private function returnNewAddressResponse(