Added nullability handling when using System.Text.Json serializer

[RSoeborg]

There is an issue handling nullable TypeId fields when deserializing with System.Text.Json

Consider the below:

public class Customer {
    public TypeIdDecoded Id { get; }
    // other props
    public TypeIdDecoded? DefaultShippingAddressId { get; }
}

When trying to deserialize the nullable property a JsonException is raised:

The JSON value could not be converted to System.Nullable`1[FastIDs.TypeId.TypeIdDecoded]. Path: $.defaultShippingAddressId

This PR aims to fix that.

[firenero]

Hi @RSoeborg, sorry for a slow response. I can't reproduce the original issue based on your example. These tests pass for me:

[TestFixture]
public class NullTests
{
    private JsonSerializerOptions _options = new JsonSerializerOptions().ConfigureForTypeId();
    
    [Test]
    public void Serialized()
    {
        var item = new Customer(TypeId.Parse("type_01h455vb4pex5vsknk084sn02q").Decode(), null);
        
        var json = JsonSerializer.Serialize(item, _options);
        
        json.Should().Be("{\"Id\":\"type_01h455vb4pex5vsknk084sn02q\",\"DefaultShippingAddressId\":null}");
    }

    [Test]
    public void Deserialized()
    {
        var json = "{\"Id\":\"type_01h455vb4pex5vsknk084sn02q\",\"DefaultShippingAddressId\":null}";
        
        var item = JsonSerializer.Deserialize<Customer>(json, _options);
        
        item.Should().BeEquivalentTo(new Customer(TypeId.Parse("type_01h455vb4pex5vsknk084sn02q").Decode(), null));
    }
    
    [Test]
    public void DeserializedWithValue()
    {
        var json = "{\"Id\":\"type_01h455vb4pex5vsknk084sn02q\",\"DefaultShippingAddressId\":\"type_01h455vb4pex5vsknk084sn02q\"}";
        
        var item = JsonSerializer.Deserialize<Customer>(json, _options);

        var expected = new Customer(
            TypeId.Parse("type_01h455vb4pex5vsknk084sn02q").Decode(),
            TypeId.Parse("type_01h455vb4pex5vsknk084sn02q").Decode()
        );
        item.Should().BeEquivalentTo(expected);
    }

    public class Customer {
        public TypeIdDecoded Id { get; }
        // other props
        public TypeIdDecoded? DefaultShippingAddressId { get; }
        
        public Customer(TypeIdDecoded id, TypeIdDecoded? defaultShippingAddressId)
        {
            Id = id;
            DefaultShippingAddressId = defaultShippingAddressId;
        }
    }
}

Could you please provide a code snipped that reproduces the error?