[confmap][fix] Correctly differentiate between nil and empty slices in ToStringMap

[mahadzaryab1]

Description

• Background
  • The confmap decoder hook has a zeroSliceHookFunc which treats empty slices provided in the config as empty slices rather than nil
  • However in confmap.ToStringMap, empty slices were getting rendered as nil which is different from the decoder hook as described above.
• This PR fixes confmap.ToStringMap by adding a check in the sanitizer function that returns an empty slice when one is provided.
• This PR is a prelude to [WIP][confmap] Enable decoding nil values in confmap #11734 to allow for the decoding of nil values in confmap to enable optional configs.

Link to tracking issue

Fixes #11749

Testing

• Adjusted the existing unit test and added a new unit test

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 91.52%. Comparing base (e91dafa) to head (5fb775c).

Files with missing lines	Patch %	Lines
confmap/confmap.go	25.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11755      +/-   ##
==========================================
- Coverage   91.53%   91.52%   -0.02%     
==========================================
  Files         443      443              
  Lines       23766    23770       +4     
==========================================
+ Hits        21754    21755       +1     
- Misses       1638     1640       +2     
- Partials      374      375       +1     

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[mahadzaryab1]

@mx-psi This block is currently not covered by tests because we're not decoding nil values in this PR (that will happen in #11734). We could (1) remove this if block here and add it in the following PR; (2) leave it in here but merge with missing code coverage; (3) add a unit test for this function directly. Let me know what you think.

[mx-psi]

Let's wait to discuss this with @songy23

[yurishkuro]

The description is confusing to me

which differs from the behaviour of confmap's decoding workflow

What does this tell me?

What I would rather see is something like this:

• the hook's description says
  • empty slice [] in yaml should override the default value assigned to config
  • but a nil slice in yaml should fall back to default value
• but in practice the code was doing this: ...

[mahadzaryab1]

The description is confusing to me

which differs from the behaviour of confmap's decoding workflow

What does this tell me?

What I would rather see is something like this:

• the hook's description says

  • empty slice [] in yaml should override the default value assigned to config
  • but a nil slice in yaml should fall back to default value

• but in practice the code was doing this: ...

@yurishkuro Thanks for the feedback! I updated the description. One thing to note is that the discrepancy is not within the hook but rather ToStringMap which is more of a helper function to create a map[string]any from a Conf. This PR matches the behaviour of ToStringMap with the behaviour of the hook and more specifically with that of zeroSliceHookFunc.