Issue #3513025: Warning: Undefined array key "display_empty".

Closes #3513025

src/Plugin/Block/MenuBlock.php

@@ -443,7 +443,7 @@ class MenuBlock extends SystemMenuBlock {
    */
   public function blockAccess(AccountInterface $account) {
     $build = $this->build();
-    if (empty($build['#items']) && !$this->configuration['display_empty']) {
+    if (empty($build['#items']) && !($this->configuration['display_empty'] ?? FALSE)) {
       return AccessResult::forbidden();
     }
     return parent::blockAccess($account);