Wrong getId type-hint, it should be nullable

[diimpp]

Hello,

Those entities won't have an ID after construction as they those IDs are assigned by doctrine, but at the same time getId/id type-hints for them are too restrictive.

--- getId(): int
+++ getId(): ?int

https://github.com/Sylius/RefundPlugin/blob/master/src/Entity/CreditMemoSequence.php
https://github.com/Sylius/RefundPlugin/blob/master/src/Entity/CustomerBillingData.php
https://github.com/Sylius/RefundPlugin/blob/master/src/Entity/LineItem.php
https://github.com/Sylius/RefundPlugin/blob/master/src/Entity/RefundPayment.php
https://github.com/Sylius/RefundPlugin/blob/master/src/Entity/ShopBillingData.php
https://github.com/Sylius/RefundPlugin/blob/master/src/Entity/TaxItem.php

LineItem and TaxItem have both getId and id methods, but CustomerBillingData and ShopBillingData doesn't, while they have similar methods schema.

I guess spec would need to be changed as well with something like

    function it_has_no_id_by_default(): void
    {
        $this->getId()->shouldReturn(null);
    }

https://github.com/Sylius/Sylius/blob/master/src/Sylius/Component/Payment/spec/Model/PaymentSpec.php#L27

Other thing that I've noticed, that LineItem and TaxItem id doctrine definition doesn't have AUTO keyword, if it means anything.
https://github.com/Sylius/RefundPlugin/blob/master/src/Resources/config/doctrine/LineItem.orm.xml#L6