[confmap] Correctly distinguish between empty and nil slices

[mx-psi]

Description

A second attempt at #11882.

Note that I added some tests in #12871 to prevent something like #12661 from happening again.

Link to tracking issue

Fixes #11749

Testing

See tests from #12871 as well as the new tests added here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.47%. Comparing base (78663f9) to head (05e91fe).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12872   +/-   ##
=======================================
  Coverage   91.47%   91.47%           
=======================================
  Files         494      494           
  Lines       26993    27006   +13     
=======================================
+ Hits        24691    24704   +13     
  Misses       1818     1818           
  Partials      484      484           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[yurishkuro]

cc @mahadzaryab1

[songy23]

LGTM