Query string starts with `&` when the first property is null

It seems that if the first property is null, the serialized output starts with &.

#!/usr/bin/env dotnet run
#:package WebSerializer@1.3.0

using System.Runtime.Serialization;
using Cysharp.Web;

var req = new Request(sortBy: null, direction: SortDirection.Desc, currentPage: 3);

var q = WebSerializer.ToQueryString(req);

//await httpClient.GetAsync("/sort?"+ q);
Console.WriteLine("/sort?"+ q);
// expected: /sort?direction=Desc&currentPage=3
// actual: /sort?&direction=Desc&currentPage=3

public record Request(
    [property: DataMember(Order = 0)] string? sortBy,
    SortDirection direction,
    int currentPage
);

public enum SortDirection
{
    Default,
    Asc,
    Desc
}

Start at the WebSerializer.ToQueryString entry point and reproduce the provided Request example, where the first property is null. Add a regression test for the expected query string without a leading ampersand, then verify the existing serialization tests still pass.