[CSharp] Bug Generating code for nullable Enum parameter does not compile

Description

When generating C# client from Open Api 3 schema where operation parameter is nullable enum with default value:

          {
            "name": "size",
            "in": "query",
            "schema": {
              "default": "Original",
              "$ref": "#/components/schemas/PhotoFileSize"
            },
            "x-position": 3
          }

....


  "components": {
    "schemas": {

      "PhotoFileSize": {
        "type": "string",
        "description": "",
        "x-enumNames": [
          "Original",
          "Avatar"
        ],
        "enum": [
          "Original",
          "Avatar"
        ]
      }
  }
}

The generated code does not set default value for the query parameter correctly and it does not define the enum parameter type nullable.

Expected result:
The parameter should be typed:
PhotoFileSize? size = null

Swagger-codegen version

swagger-codegen-cli-3.0.23

Swagger declaration file content or url

Command line used for generation

swagger-codegen-cli-3.0.23

Steps to reproduce

Related issues/PRs

Suggest a fix/enhancement

Run swagger-codegen-cli-3.0.23 with the OpenAPI 3 schema shown in the report, focusing on the nullable enum query parameter with default value "Original". Compare the generated C# signature with the expected PhotoFileSize? size = null result; done when the generated parameter is nullable, preserves the default value, and compiles.