Fix PHP 8.4 deprecation

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

lib/CacheInterface.php

@@ -4,7 +4,7 @@ interface CacheInterface
 {
     public function get(string $key, $default = null);
 
-    public function set(string $key, $value, int $ttl = null): void;
+    public function set(string $key, $value, ?int $ttl = null): void;
 
     public function delete(string $key): void;