Disable cookie authentication if credentials are missing.

[smithsz]

Checklist

• [x] Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• [x] Added tests for code changes or test/build only changes
• [x] Updated the change log file (CHANGES.md|CHANGELOG.md) or test/build only changes
• [x] Completed the PR template below:

Description

Disable cookie authentication if credentials are missing.

Approach

If credentials are missing then set a 'disabled' property on the cookie plugin to prevent hooks from being executed.

Schema & API Changes

No change.

Security and Privacy

No change.

Testing

Includes additional unit tests.

Monitoring and Logging

No change.