fix: handle nullable enums in experimental parser

[mrlubos]

Closes #1246

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🦋 Changeset detected

Latest commit: 61cd848

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@hey-api/openapi-ts	Patch

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Name	Status	Preview	Comments	Updated (UTC)
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[codecov]

Codecov Report

Attention: Patch coverage is 40.32258% with 37 lines in your changes missing coverage. Please review.

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Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
...ages/openapi-ts/src/openApi/3.0.x/parser/schema.ts	0.00%	30 Missing ⚠️
...ages/openapi-ts/src/openApi/3.1.x/parser/schema.ts	77.77%	6 Missing ⚠️
packages/openapi-ts/src/openApi/index.ts	80.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1248   +/-   ##
=======================================
  Coverage        ?   67.69%           
=======================================
  Files           ?      132           
  Lines           ?    19491           
  Branches        ?     1749           
=======================================
  Hits            ?    13195           
  Misses          ?     6292           
  Partials        ?        4           

Flag	Coverage Δ
unittests	67.69% <40.32%> (?)

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