fix: hide backoffice navbar-toggler when there's nothing to collapse

The toggler button was always rendered, but its collapse target
(#navbarSupportedContent) only existed for authenticated users. On
the login page, tapping the menu button on a narrow screen did
nothing because the target it pointed at wasn't in the DOM.

templates/backoffice/nav.html.twig

@@ -1,16 +1,16 @@
 <nav class="navbar navbar-expand-lg bg-body-tertiary">
     <div class="container-fluid">
         <a class="navbar-brand" href="{{ path('tvdt_backoffice_index') }}">Tijd voor de test</a>
-        <button class="navbar-toggler"
-                type="button"
-                data-bs-toggle="collapse"
-                data-bs-target="#navbarSupportedContent"
-                aria-controls="navbarSupportedContent"
-                aria-expanded="false"
-                aria-label="Toggle navigation">
-            <span class="navbar-toggler-icon"></span>
-        </button>
         {% if is_granted('IS_AUTHENTICATED') %}
+            <button class="navbar-toggler"
+                    type="button"
+                    data-bs-toggle="collapse"
+                    data-bs-target="#navbarSupportedContent"
+                    aria-controls="navbarSupportedContent"
+                    aria-expanded="false"
+                    aria-label="Toggle navigation">
+                <span class="navbar-toggler-icon"></span>
+            </button>
             <div class="collapse navbar-collapse" id="navbarSupportedContent">
                 <ul class="navbar-nav me-auto mb-2 mb-lg-0">
                     <li class="nav-item">

tests/Controller/LoginControllerTest.php

@@ -19,6 +19,24 @@ final class LoginControllerTest extends AbstractControllerWebTestCase
         self::assertSelectorExists('form');
     }
 
+    public function testNavbarTogglerHasNoDeadTargetWhenNotAuthenticated(): void
+    {
+        $this->client->request(Request::METHOD_GET, '/login');
+
+        $crawler = $this->client->getCrawler();
+        $toggler = $crawler->filter('.navbar-toggler');
+
+        if (0 === $toggler->count()) {
+            self::assertSelectorNotExists('#navbarSupportedContent');
+
+            return;
+        }
+
+        $target = $toggler->attr('data-bs-target');
+        $this->assertNotNull($target);
+        self::assertSelectorExists($target);
+    }
+
     public function testLoginRedirectsToBackofficeWhenAlreadyAuthenticated(): void
     {
         $this->loginAs('test@example.org');