Fix PHP 8.4 deprecation

Implicitly marking parameter as nullable is deprecated, the explicit nullable type must be used instead

caches/NullCache.php

@@ -9,7 +9,7 @@ class NullCache implements CacheInterface
         return $default;
     }
 
-    public function set(string $key, $value, int $ttl = null): void
+    public function set(string $key, $value, ?int $ttl = null): void
     {
     }