UserController: Behave nicely when not any backend is available

application/controllers/UserController.php

@@ -28,6 +28,10 @@ class UserController extends AuthBackendController
             function ($b) { return $b->getName(); },
             $this->loadUserBackends('Icinga\Data\Selectable')
         );
+        if (empty($backendNames)) {
+            return;
+        }
+
         $this->view->backendSelection = new Form();
         $this->view->backendSelection->setAttrib('class', 'backend-selection');
         $this->view->backendSelection->setUidDisabled();

application/views/scripts/user/list.phtml

@@ -18,7 +18,7 @@ if (! $this->compact): ?>
 <div class="content users">
 <?php
 
-if ($backend === null) {
+if (! isset($backend)) {
     echo $this->translate('No backend found which is able to list users') . '</div>';
     return;
 } else {