Allow null Session in CallbackSession when there is an error

[jsardev]

This is a follow-up of #180.

The change allows to call the SessionStore.get callback parameter with

callback(new Error());

instead of the current unnecessary

callback(new Error(), null)

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[jsardev]

Oh, nevermind. I guess this has been already fixed by #190. I think both this and #180 can be closed.