Nullable types must be explicit.

Closes #3506354

src/Access/MenuAdminPerMenuAccess.php

@@ -98,7 +98,7 @@ class MenuAdminPerMenuAccess implements ContainerInjectionInterface, MenuAdminPe
   /**
    * {@inheritdoc}
    */
-  public function menuItemAccess(AccountInterface $account, MenuLinkContent $menu_link_content = NULL) {
+  public function menuItemAccess(AccountInterface $account, ?MenuLinkContent $menu_link_content = NULL) {
     if (!$menu_link_content instanceof MenuLinkContent) {
       return AccessResult::neutral();
     }
@@ -114,7 +114,7 @@ class MenuAdminPerMenuAccess implements ContainerInjectionInterface, MenuAdminPe
   /**
    * {@inheritdoc}
    */
-  public function menuLinkAccess(AccountInterface $account, MenuLinkInterface $menu_link_plugin = NULL) {
+  public function menuLinkAccess(AccountInterface $account, ?MenuLinkInterface $menu_link_plugin = NULL) {
     if (!$menu_link_plugin instanceof MenuLinkInterface) {
       return AccessResult::neutral();
     }