fix(conn): better tolerate not-yet-known ClientID when emitting stats

Release version: v5.8.3.

Summary

See individual commits.

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

[x] Added tests for the changes
[ ] ~~Added property-based tests for code which performs user input validation~~
[ ] Changed lines covered in coverage report
[ ] Change log has been added to changes/(ce|ee)/(feat|perf|fix|breaking)-<PR-id>.en.md files
[x] For internal contributor: there is a jira ticket to track this change
[ ] ~~Created PR to emqx-docs if documentation update is required, or link to a follow-up jira ticket~~
[x] Schema changes are backward compatible