Make apcu backend explicitly nullable

src/CryptologMiddleware.php

@@ -39,7 +39,7 @@ class CryptologMiddleware implements CryptologMiddlewareInterface, HttpKernelInt
   public function __construct(
     protected HttpKernelInterface $httpKernel,
     protected CacheBackendInterface $cacheBackend,
-    ApcuBackendFactory $apcuBackendFactory = NULL,
+    ?ApcuBackendFactory $apcuBackendFactory = NULL,
     protected ?int $ttl = NULL,
   ) {
     // Allow single-webhead sites to avoid database storage by injecting the