Missing nullable property for nullable reference type dictionary values

[ben-kiplot]

NSwag seems unable to correctly generate API clients for dictionary properties with nullable reference type values, such as public Dictionary<string, object?> { get; set; } and I think I've tracked this down to an issue with NJsonSchema not supporting this.

Generating a schema for this class:

public class TestDto
{
    public required Dictionary<string, object?> TestProperty { get; set; };
}

incorrectly generates the following schema which lacks the nullable additional property:

"TestDto": {
  "type": "object",
  "additionalProperties": false,
  "required": [
    "testProperty"
  ],
  "properties": {
    "testProperty": {
      "type": "object",
      "additionalProperties": {}
    }
  }
}

Nullable primitives don't exhibit the same issue however:

public class TestDto
{
    public required Dictionary<string, string?> TestProperty { get; set; };
}

correctly generates the following:

"TestDto": {
  "type": "object",
  "additionalProperties": false,
  "required": [
    "testProperty"
  ],
  "properties": {
    "testProperty": {
      "type": "object",
      "additionalProperties": {
        "type": "string",
        "nullable": true
      }
    }
  }
}

I've made a small change that shows when the schema generation is changed to set the property as nullable the C# code generation does correctly generate a Dictionary<string, object?> as expected. The commit is here: 5f52e9d

This change doesn't result in the exact same code as shown above for nullable primitives though so perhaps this isn't the correct way to fix this.

"NullableObjectDictionary": {
      "type": "object",
      "additionalProperties": {
        "x-nullable": true
      }
    }