Issue#3363469: Resolve string php deprecation.

src/AccessChecks/RouteAccessChecks.php

@@ -34,7 +34,7 @@ class RouteAccessChecks implements AccessInterface {
     }
 
     // Get a list of the restricted routes from config
-    $config_route_paths = $config->get('menu_custom_access.routes');
+    $config_route_paths = $config->get('menu_custom_access.routes') ?? '';
     $route_paths = preg_split('/\r\n|\r|\n/', $config_route_paths);
     $config_roles = $config->get('menu_custom_access.roles');
     $account_has_role = array_intersect($account->getRoles(), $config_roles);