fix(health): treat nil return from Lua health status script as n/a instead of unknown

[snorwin]

Custom Lua health check scripts currently have no way to indicate that no health status should apply to a resource. This is extremely useful for generic health checks covering multiple resources in an API group, where some resources shouldn't report a health status at all.

Actual behavior
return nil from a Lua health check script results in the resource's health status being reported as Unknown.

Expected behavior
return nil should result in the health status being absent, i.e., equivalent to there being no health check defined for the object.

Note: As far as I've checked, none of the currently built-in health check scripts would be affected by this change.

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Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Title of the PR
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
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• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

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[codecov]

Bundle Report

Bundle size has no change ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 64.81%. Comparing base (38095fc) to head (90788d6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #28269      +/-   ##
==========================================
- Coverage   64.82%   64.81%   -0.02%     
==========================================
  Files         425      425              
  Lines       59092    59092              
==========================================
- Hits        38306    38299       -7     
- Misses      17222    17233      +11     
+ Partials     3564     3560       -4     

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