How to ignore null properties when serializing to JSON in middleware?

In ASP.NET Core Web API I want to ignore null properties, when returning the following DTO class (I'm using System.Text.Json;, not Newtonsoft.Json). I want to do it in middleware (in Startup class), not in Controller. Here's the C# class:

public class VehicleOutput
{
    public int VehicleId { get; set; }
    public string Owner { get; set; }

    public Car Car { get; set; }
    public Truck Truck { get; set; }
}

This is JSON I'd like to return (I don't want to show "car": null if Car in fact is null, and same for Truck):

[
    {
        "vehicleId": 1,
        "owner": "ABS",
        "car": 
        {
            "carId": 123,
            "model": "Foo"
        }
    },
    {
        "vehicleId": 2,
        "owner": "ABS",
        "truck": 
        {
            "truckId": 123,
            "model": "Bar"
        }
    }
]

I was able to achive that by doing this in the Controller. It works!:

    [HttpGet]
    public async Task<ActionResult> GetVehicles()
    {
        var vehiclesOutput = await _vehicleService.GetVehicles();

        var options = new JsonSerializerOptions
        {
            IgnoreNullValues = true,
            WriteIndented = true
        };

        return Ok(JsonSerializer.Serialize(vehiclesOutput, options));
    }

But I don't want to repeat this code every time I return VehicleOutput class. So I want to do it in middleware. This is what I got:

        services.AddControllers()
           .AddJsonOptions(opt =>
           {
               opt.JsonSerializerOptions.IgnoreNullValues = true;
           });

It works! It ignores null properties in JSON, but it does it for every object I'm returning in the API. I want to add this option ONLY for specific classes (like VehicleOutput). How can you do that in Startup class?

Bear in mind... I'm not using Newtonsoft.Json. I'm using System.Text.Json; and I want to stick with it (if possible)