Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Bounty to sponsor the fix (example)

Description

In version 4.0.3, when using an OpenAPI 2.0 input, model properties were generated as nullable. For example, see that the date-time property in the FormatTest class was generated as a DateTime? type in 4.0.3:
https://github.com/OpenAPITools/openapi-generator/blob/v4.0.3/samples/client/petstore/csharp/OpenAPIClient/src/Org.OpenAPITools/Model/FormatTest.cs#L172

But in 4.1.0, the same property is generated as a non-nullable DateTime type:
https://github.com/OpenAPITools/openapi-generator/blob/v4.1.0/samples/client/petstore/csharp/OpenAPIClient/src/Org.OpenAPITools/Model/FormatTest.cs#L172

This is a problem because if a service returns null for these properties in a response, the JSON deserialization fails.

Is there a way to generate nullable properties using an OpenAPI 2.0 input?

openapi-generator version

4.1.0

OpenAPI declaration file content or url

https://github.com/OpenAPITools/openapi-generator/blob/v4.1.0/modules/openapi-generator/src/test/resources/2_0/petstore-with-fake-endpoints-models-for-testing.yaml#L1301

Command line used for generation

See https://github.com/OpenAPITools/openapi-generator/blob/v4.1.0/bin/csharp-petstore-netcore-project.sh#L30

Steps to reproduce

Look at the generated samples in Github for version 4.0.3 and 4.1.0 and compare the types of the properties in the generated models.

Related issues/PRs

I believe this may have been introduced by these pull requests:
#3530
#3537

Suggest a fix

I think the changes made in the above pull requests should only affect OpenAPI 3.0 generation.