fix: temporary workaround for #1821

[shaneutt]

@clux, for your considerations this is an interim solution for #1821. Only after I finished this did I see there was another PR. This approach is very focused, matching the "empty" extra obj, and removing it when found. This also adds a regression test for Optional<Enum> behavior.

needs #1842

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.7%. Comparing base (b14f3d7) to head (2316532).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1841     +/-   ##
=======================================
+ Coverage   74.6%   74.7%   +0.1%     
=======================================
  Files         84      84             
  Lines       7910    7931     +21     
=======================================
+ Hits        5900    5923     +23     
+ Misses      2010    2008      -2     

Files with missing lines	Coverage Δ
kube-core/src/schema.rs	96.1% <100.0%> (+1.0%)	⬆️
kube-derive/tests/crd_schema_test.rs	100.0% <100.0%> (ø)

... and 3 files with indirect coverage changes

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[Danil-Grigorev]

I think this is a good patch to fix the issue. I recall trying once to use some of the schemars options like AddNullable transform as recommended in https://github.com/GREsau/schemars/blob/master/CHANGELOG.md#changed-3 but it didn’t have an effect.