chore: adjusts coding style

src/PendingCalls/AfterEachCall.php

@@ -35,7 +35,7 @@ final class AfterEachCall
     public function __construct(
         private readonly TestSuite $testSuite,
         private readonly string $filename,
-        Closure $closure = null
+        ?Closure $closure = null
     ) {
         $this->closure = $closure instanceof Closure ? $closure : NullClosure::create();