make clock argument of InMemoryCache constructor explicitly nullable (#4

)

src/InMemoryCache.php

@@ -17,7 +17,7 @@ final class InMemoryCache implements CacheItemPoolInterface
     private array $deferredItems;
 
     public function __construct(
-        ClockInterface $clock = null,
+        ?ClockInterface $clock = null,
     ) {
         $this->clock = $clock ?? new class implements ClockInterface {
             public function now(): DateTimeImmutable