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">