[BUG] [aspnetcore] Nullable annotations work incorrectly with nested objects

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

The generator produces invalid source code (missing nullable type annotation ? ) for models that contain nullable nested objects

openapi-generator version

openapi-generator-cli version : 5.3.0

OpenAPI declaration file content or url

openapi: 3.0.1

info:
  title: test-api
  version: 1.0.0

paths:
  "/":
    get:
      operationId: get
      summary: Get test data
      responses:
        200:
          description: Test data
          content:
            application/json:
              schema:
                type: object
                title: Data
                properties:
                  subObject:
                    type: object
                    nullable: true
                    properties:
                      name:
                        type: string
                  name:
                    type: string
                    nullable: true

Generation Details

The following command is used to generate source code:

java -jar openapi-generator-cli.jar generate -g aspnetcore --model-name-suffix Model -c aspnetcore-config.json -i spec.yaml -o generated

and aspnetcore-config.json contains the following data:

{
    "aspnetCoreVersion": "5.0",
    "nullableReferenceTypes": true,
    "packageName": "Test"
}

Steps to reproduce

1. Create a specification file and name it spec.yaml
2. Create a configuration file and name it apsnetcore-config.json
3. Run the command (see generation details)
4. Make sure that generated/src/Test/Models/DataModel.cs contains an incorrect model:

/// <summary>
/// Gets or Sets SubObject
/// </summary>
[DataMember(Name="subObject", EmitDefaultValue=true)]
public DataSubObjectModel SubObject { get; set; }

/// <summary>
/// Gets or Sets Name
/// </summary>
[DataMember(Name="name", EmitDefaultValue=true)]
public string? Name { get; set; }

but expected (missing ? after DataSubObjectModel):

/// <summary>
/// Gets or Sets SubObject
/// </summary>
[DataMember(Name="subObject", EmitDefaultValue=true)]
public DataSubObjectModel? SubObject { get; set; }

/// <summary>
/// Gets or Sets Name
/// </summary>
[DataMember(Name="name", EmitDefaultValue=true)]
public string? Name { get; set; }

Related issues/PRs

There is a merged PR that adds support for nullable reference types: https://github.com/OpenAPITools/openapi-generator/pull/9620

Reproduce the issue with spec.yaml, aspnetcore-config.json, and the documented Java CLI command, then inspect generated/src/Test/Models/DataModel.cs. Compare the generated nested SubObject property with the expected C# output and verify that nullable nested objects receive the ? annotation without changing the already-correct Name property.