Update ItemInterface.php

phpdoc for ItemInterface::lock() should probably be ?int not null.

src/Stash/Interfaces/ItemInterface.php

@@ -88,7 +88,7 @@ public function isMiss(): bool;
      * Enables stampede protection by marking this specific instance of the Item
      * as the one regenerating the cache.
      *
-     * @param  null $ttl
+     * @param  ?int $ttl
      * @return bool
      */
     public function lock(int $ttl = null): bool;