Fix value merging for empty lists

[blampe]

This fixes our behavior to match upstream's when merging empty lists. We now produce an empty list instead of a no-op.

The bug was due to this logic which initialized a slice as nil. This was then passed to mergo.Merge which ignores zero values by default, hence why the original value was not overridden.

This PR changes our allowNullValues logic to use upstream's merge behavior more directly since the intention behind that flag is to match upstream as closely as possible.

Importantly, this doesn't fix #2034 because we're still unmarshaling inputs with SkipNulls: true. In practice allowNullValues behaves more like allowEmptyValues because our unmarshaling strips away all of the meaningful nil values. That fix will be in another PR.

Fixes #2731.
Refs #2034.
Refs #2963.

[github-actions]

Does the PR have any schema changes?

Looking good! No breaking changes found.
No new resources/functions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 35.82%. Comparing base (04fb15c) to head (12ffc99).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2995      +/-   ##
==========================================
- Coverage   35.97%   35.82%   -0.15%     
==========================================
  Files          70       70              
  Lines        9156     9135      -21     
==========================================
- Hits         3294     3273      -21     
  Misses       5530     5530              
  Partials      332      332              

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