Controlled generation JSON output with null values

public async Task<string> GenerateContentWithResponseSchema3(
    string projectId = "your-project-id",
    string location = "us-central1",
    string publisher = "google",
    string model = "gemini-2.0-flash-001")
{

    var predictionServiceClient = new PredictionServiceClientBuilder
    {
        Endpoint = $"{location}-aiplatform.googleapis.com"
    }.Build();

    var responseSchema = new OpenApiSchema
    {
        Type = Type.Object,
        Properties =
        {
            ["forecast"] = new()
            {
                Type = Type.Array,
                Items = new()
                {
                    Type = Type.Object,
                    Properties =
                    {
                        ["Forecast"] = new() { Type = Type.String },
                        ["Humidity"] = new() { Type = Type.String },
                        ["Temperature"] = new() { Type = Type.Integer },
                        ["Wind Speed"] = new() { Type = Type.Integer }
                    }
                }
            }
        }
    };

    string prompt = @"
    The week ahead brings a mix of weather conditions.
    Sunday is expected to be sunny with a temperature of 77°F and a humidity level of 50%. Winds will be light at around 10 km/h.
    Monday will see partly cloudy skies with a slightly cooler temperature of 72°F and humidity increasing to 55%. Winds will pick up slightly to around 15 km/h.
    Tuesday brings rain showers, with temperatures dropping to 64°F and humidity rising to 70%. Expect stronger winds at 20 km/h.
    Wednesday may see thunderstorms, with a temperature of 68°F and high humidity of 75%. Winds will be gusty at 25 km/h.
    Thursday will be cloudy with a temperature of 66°F and moderate humidity at 60%. Winds will ease slightly to 18 km/h.
    Friday returns to partly cloudy conditions, with a temperature of 73°F and lower humidity at 45%. Winds will be light at 12 km/h.
    Finally, Saturday rounds off the week with sunny skies, a temperature of 80°F, and a humidity level of 40%. Winds will be gentle at 8 km/h.";

    var generateContentRequest = new GenerateContentRequest
    {
        Model = $"projects/{projectId}/locations/{location}/publishers/{publisher}/models/{model}",
        Contents =
        {
            new Content
            {
                Role = "USER",
                Parts =
                {
                    new Part { Text = prompt }
                }
            }
        },
        GenerationConfig = new GenerationConfig
        {
            ResponseMimeType = "application/json",
            ResponseSchema = responseSchema
        },
    };

    GenerateContentResponse response = await predictionServiceClient.GenerateContentAsync(generateContentRequest);

    string responseText = response.Candidates[0].Content.Parts[0].Text;
    Console.WriteLine(responseText);

    return responseText;
}