Issue #3259807 by idebr, anneke_vde: #type => 'toolbar_item' without a tab...

Issue #3259807 by idebr, anneke_vde: #type => 'toolbar_item' without a tab triggers a deprecation notice on PHP8.1

(cherry picked from commit b9e078a0)

core/modules/toolbar/src/Element/ToolbarItem.php

@@ -25,7 +25,7 @@ public function getInfo() {
       ],
       'tab' => [
         '#type' => 'link',
-        '#title' => NULL,
+        '#title' => '',
         '#url' => Url::fromRoute('<front>'),
       ],
     ];

core/modules/toolbar/tests/modules/toolbar_test/toolbar_test.module

@@ -45,6 +45,9 @@ function toolbar_test_toolbar() {
     ],
     '#weight' => 50,
   ];
+  $items['empty'] = [
+    '#type' => 'toolbar_item',
+  ];
 
   return $items;
 }