[confmap] Fix handling for slices with default values

[evan-bradley]

Description

#11882 introduced issues with handling slices which have default values. It's likely that the removal of zeroSliceHookFunc is causing problems with overriding default lists.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.63%. Comparing base (8bc15fd) to head (a379cff).
Report is 1 commits behind head on release/v0.122.x.

Additional details and impacted files

@@                Coverage Diff                @@
##           release/v0.122.x   #12662   +/-   ##
=================================================
  Coverage             91.63%   91.63%           
=================================================
  Files                   479      479           
  Lines                 26423    26423           
=================================================
  Hits                  24214    24214           
  Misses                 1749     1749           
  Partials                460      460           

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