fix variables not nullable in request callback typescript definition

[tomdardev]

The RequestCallback type definition didn't mention that both of its parameters are nullable. This is especially confusing since the documentation explicitly mentions the response parameter as being nullable.

Checklist

• only relevant code is changed (make a diff before you submit the PR)
• run tests npm run test
• tests are included
• commit message and code follows the Developer's Certification of Origin

[adrai]

thank you... will be included in the next release