Allow Payment Notes to be null

This PR allows null to be passed as a payment note. There might be more PRs like this as I find the lesser used CBS fields and make them less restrictive, but I'm going to merge these ASAP since they're just annotations and I'm doing some testing.

Notify: @ahoffmann @weizhong-wang @lloyd-carter @KJOYNER

merged

mentioned in commit 120d7e87

What happens if you pass an empty string?

@ahoffmann that would have worked but we would have to check what the Magento order is returning (either a string or null) and send '' if the Magento order returned null. Allowing null straight away was less logic.

@andrew-summers that makes sense, thanks!