Order user can be null

After https://github.com/Sylius/Sylius/pull/1816 `order.user` can be null.

So, all code like `$order->getUser()->` should be fixed to support this change.

For example https://github.com/Sylius/Sylius/blob/master/src/Sylius/Bundle/PayumBundle/Payum/Be2bill/Action/CapturePaymentUsingBe2billFormAction.php#L70.
This looks wrong, it should be probably:
```php
$details['CLIENTIDENT'] = $order->getUser() ? $order->getUser()->getId() : $order->getEmail();