CSharpNetCoreClientCodegen generate nullable TimeSpan

Description

Hi guys. Is there possibility generate nullable TimeSpan from scheme. I have --type-mappings arg in command line. It it map type correctly but nullable description is not involved.

Thank you for your help!

openapi-generator version

4.22

OpenAPI declaration file content or url

 "startTime": {
            "type": "string",
            "format": "date-span"
          },
          "endTime": {
            "type": "string",
            "format": "date-span",
            "nullable": true
          }, ....

Command line used for generation

openapi-generator generate -g csharp-netcore -i http://localhost:22525/swagger/v1/swagger.json -o RestApiClients -c v1-config.json -DapiTests=false -DapiDocs=false -DmodelTests=false -DmodelDocs=false --type-mappings date-span=TimeSpan --package-name Rest.V1

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement

Reproduce the issue with the csharp-netcore generator, the supplied OpenAPI schema, and the shown --type-mappings command. Start by tracing how the generator handles the date-span format and nullable property, then verify that the generated model represents the nullable field as TimeSpan and that the non-nullable field remains unchanged.