Serialization issue when property is set to null

ExtendedXmlSerializer.cs

namespace ConsoleApplication1
{
    class Program
    {
        public class Foo
        {
            public string MyStr { get; set; }
        }

        static void Main(string[] args)
        {
            var serializer = new ExtendedXmlSerializer();

            Console.WriteLine("Missing MyStr node in XML:");
            Console.WriteLine(serializer.Serialize(new Foo { MyStr = null }));

            Console.WriteLine("----");

            Console.WriteLine("Constains MyStr node in XML");
            Console.WriteLine(serializer.Serialize(new Foo { MyStr = "" }));

            Console.ReadLine();
        }
    }
}