Mark enum as nullable for null enum members

[aburgel]

Fixes #1030

According to the 3.0 docs and this FAQ, an enum is nullable when both it's type is nullable and when null is a member of the enum. Currently kubb will crash with a null member, so this PR fixes this case.

I added a couple of test cases as well, but let me know if there are more ways I should test this.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 61ad74ebd96f8ee10dfcc34d7e5f40cfa9dd0400

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

This PR includes changesets to release 23 packages

| Name | Type | | ---------------------------- | ----- | | @kubb/plugin-oas | Patch | | @kubb/cli | Patch | | @kubb/plugin-redoc | Patch | | @kubb/swagger-client | Patch | | @kubb/swagger-faker | Patch | | @kubb/swagger-msw | Patch | | @kubb/swagger-swr | Patch | | @kubb/swagger-tanstack-query | Patch | | @kubb/swagger-ts | Patch | | @kubb/swagger-zod | Patch | | @kubb/swagger-zodios | Patch | | @kubb/swagger | Patch | | @kubb/config-biome | Patch | | @kubb/config-ts | Patch | | @kubb/config-tsup | Patch | | @kubb/core | Patch | | @kubb/fs | Patch | | @kubb/oas | Patch | | @kubb/parser-ts | Patch | | @kubb/react | Patch | | @kubb/types | Patch | | kubb | Patch | | unplugin-kubb | Patch |

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[stijnvanhulle]

@all-contributors add @aburgel for code

[allcontributors[bot]]

@stijnvanhulle

I've put up a pull request to add @aburgel! :tada: