[openapi-normalizer] Support normalizing null types using OAS 3.1 syntax

[karzang]

In some test specifications OAS 3.1 syntax is used to definenullable schemas in oneOf/anyOf. This change supports normalizing these definitions and avoids generating unnecessary code for null types using the SIMPLIFY_ONEOF_ANYOF option.

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@wing328

[wing328]

Thanks for the PR but your commit (as shown in the Commits tab) is not linked to your Github account, which means this PR won't count as your contribution in https://github.com/OpenAPITools/openapi-generator/graphs/contributors.

Let me know if you need help fixing it.

Ref: https://github.com/OpenAPITools/openapi-generator/wiki/FAQ#how-can-i-update-commits-that-are-not-linked-to-my-github-account

[karzang]

I added the E-Mail in the commit to my Github account. According to your description that should fix the problem.