[14.x] Allow guest checkout

[alexanderkroneis]

This PR introduces a simple change of $owner in Checkout being nullable. This allows developers to implement a anonymous checkout / guest checkout for their applications.

Checkout::anonymous($sessionOptions);

[alexanderkroneis]

I just recognized that there will be a null pointer exception and it seems this implementation is not possible.

[driesvints]

It's possible. I think this is a good idea. Let me push some changes.

[driesvints]

I renamed it to Checkout::guest($sessionOptions);.

[driesvints]

I'm gonna postpone releasing this. I'm working on a few improvements but ain't entirely happy yet with the implementation: #1439