fix: CheckoutManager::getNextStep return type

`CoreShop\Component\Order\Checkout\CheckoutManager::getNextStep(): Return value must be of type CoreShop\Component\Order\Checkout\CheckoutStepInterface, null returned`

src/CoreShop/Component/Order/Checkout/CheckoutManager.php

@@ -51,7 +51,7 @@ public function getStep(string $identifier): CheckoutStepInterface
         return $step;
     }
 
-    public function getNextStep(string $identifier): CheckoutStepInterface
+    public function getNextStep(string $identifier): ?CheckoutStepInterface
     {
         return $this->serviceRegistry->getNextTo($identifier);
     }