fix: release checker when nodes are nil

[Revolyssup]

Follow up: https://github.com/apache/apisix/pull/12126/files
Fixes # The case when nodes are nil still doesn't release healthchecker. This PR fixes that.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[Baoyuantop]

Hi @Revolyssup, is this PR still being processed?

[Revolyssup]

@Baoyuantop I am waiting for review from my side.

[github-actions]

This pull request has been marked as stale due to 60 days of inactivity. It will be closed in 4 weeks if no further activity occurs. If you think that's incorrect or this pull request should instead be reviewed, please simply write any comment. Even if closed, you can still revive the PR at any time or discuss it on the [email protected] list. Thank you for your contributions.

[github-actions]

This pull request has been marked as stale due to 60 days of inactivity. It will be closed in 4 weeks if no further activity occurs. If you think that's incorrect or this pull request should instead be reviewed, please simply write any comment. Even if closed, you can still revive the PR at any time or discuss it on the [email protected] list. Thank you for your contributions.

[Baoyuantop]

Hi @Revolyssup, do we still need this PR?

[nic-6443]

@Baoyuantop we can close it now, we've completely refactored the health checker module.