Serializes POCO to XML. Converts it to Json with Json.NET. Parses it to a JObject and queries it with SelectToken. This turns an empty string to null.

gistfile1.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;
using System.Xml;
using System.Xml.Serialization;
using CSScriptLibrary;
using Newtonsoft.Json;
using Newtonsoft.Json.Linq;

namespace ConsoleApplication1
{
	[Serializable]
	public class Serializetest
	{
		public Serializetest()
		{
			Empty = string.Empty;
		}

		public string Empty { get; set; }
	}

	class Program
	{
		static void Main(string[] args)
		{
			var serializetest = new Serializetest();
			string xmlText;

			var serializer = new XmlSerializer(typeof(Serializetest));
			using (StringWriter textWriter = new StringWriter())
			{
				using (XmlWriter xmlWriter = XmlWriter.Create(textWriter))
				{
					serializer.Serialize(xmlWriter, serializetest);
				}
				xmlText = textWriter.ToString();
			}
			
			var doc = new XmlDocument();
			doc.LoadXml(xmlText);
			var jsonText = JsonConvert.SerializeXmlNode(doc);

			var xmlFromJson = JsonConvert.DeserializeXmlNode(jsonText);
			// Back to XML it's "" again.

			var objectFromJson = JsonConvert.DeserializeObject<Serializetest>(jsonText);
			// Correctly deserialized to ""

			// I want to access the Json directly
			var json = JObject.Parse(jsonText);
			var empty = (string)json.SelectToken("Serializetest.Empty");
			// null, this is the problem.
			
			var dynamicObj = JsonConvert.DeserializeObject<dynamic>(jsonText);
			var t = (string)dynamicObj.Serializetest.Empty;
			// t is also null
		}
	}
}

http://stackoverflow.com/questions/24368006/how-to-get-json-net-to-serialize-empty-xml-element-to-empty-string