reading xml file error : Data at the root level is invalid. Line 1, position 1

<?xml version="1.0" encoding="utf-8"?>
<root />

i have an empty xml file wish i created using this function

 public static void CreateXmlFile(string fileName, string rootElementName)
{
    XmlTextWriter writer = new XmlTextWriter(fileName, System.Text.Encoding.UTF8);
   writer.WriteStartDocument();
    writer.Formatting = Formatting.Indented;
    writer.Indentation = 2;
    writer.WriteStartElement(rootElementName);
    writer.WriteEndElement();
    writer.WriteEndDocument();
    writer.Close();
}

when I am trying to read it and converting it to DataTable it raise this error : Data at the root level is invalid. Line 1, position 1. can anyone tell me how can i fix this problem

 public static DataTable ConvertXMLToDataTable(string xmlString)
{
    DataSet dataset = new DataSet();
    System.Xml.XmlTextReader reader = new System.Xml.XmlTextReader(new System.IO.MemoryStream(System.Text.UTF8Encoding.Default.GetBytes(xmlString)));
    dataset.ReadXml(reader);
    return dataset.Tables[0];
}

 string fileName = "Chat_" + UserID + "_" + FriendID+".xml";
        string path = HttpContext.Current.Server.MapPath("~/PrivateChatXmls/" + fileName);
        if (!File.Exists(path))
            CreateXmlFile(path, "root");
        else
        {
            DataTable Chat_dt = Convertor.ConvertXMLToDataTable(path);
            .................