magento2: $order->getCustomer() returns NULL for registered customer

Preconditions (*)

1. Magento 2.3.2 & 2.4-develop, community edition

Steps to reproduce (*)

1. Create customer account and make an order
2. Create a module with an observer to sales_order_save_after event

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Event/etc/events.xsd">
    <event name="sales_order_save_after">
        <observer name="my-observer" instance="Something\Darkside\Observer\OrderSaveAfter" />
    </event>
</config>

1. Try to access Customer object from order object

public function execute(Observer $observer): void
    {
         /** @var Order $order */
         $order = $observer->getEvent()->getOrder();
         $customer = $order->getCustomer();
        ....

Expected result (*)

1. Method getCustomer should return customer object, as defined in phpdoc block OR phpdoc block should be changed to highlight that NULL could be returned

Actual result (*)

1. Method getCustomer always returns NULL, but it’s not highlighted in phpdoc block that it could return null

Additional info

Method getCustomer will return customer only in case if someone added it to Order object, in other cases it doesn’t exists. In case if you need to get customer from the Order object you need to do following get customer ID from the order object

$customerId = $order->getCustomerId();

// $customerId could be NULL in case if order was placed as guest
$customer = $customerId ? $this->customerRepository->getById($customerId) : null;

Where $this->customerRepository is instance of \Magento\Customer\Api\CustomerRepositoryInterface that retrieved via constructor