Prevent auth_token from being overwritten with None

Summary

Addresses https://gitlab.indiscale.com/caosdb/customers/f-fit/management/-/issues/82 / https://gitlab.com/linkahead/linkahead-pylib/-/issues/103
The auth_token is no longer overwritten with None in authentication/interface/on_response(), preventing the AuthTokenAuthenticator from becoming invalid when a request does not provide a new token in the "Set-Cookie" header.
Test MR is here.

Test Environment

Manual Testing

Check List for the Author

• All automated tests pass
• Reference related issues
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
• Annotations in code (Gitlab comments)

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md (or not necessary)
• Appropriate user and developer documentation (or not necessary)
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?