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?

CHANGELOG.md

@@ -16,6 +16,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Removed ###
 
 ### Fixed ###
+* [#103](https://gitlab.com/linkahead/linkahead-pylib/-/issues/103)
+  `authentication/interface/on_response()` does not overwrite
+  `auth_token` if new value is `None`
 
 ### Security ###
 
@@ -47,7 +50,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed ###
 
-* [gitlab.indiscale.com#200](https://gitlab.indiscale.com/caosdb/src/caosdb-pylib/-/merge_requests/153)
+* [gitlab.indiscale.com#200](https://gitlab.indiscale.com/caosdb/src/caosdb-pylib/-/issues/200)
   ``linkahead_admin.py`` prints reasonable error messages when users
   or roles don't exist.