Default Transcoder class - DecodeString method doesn't manage null string

protected string DecodeString(byte[] buffer, int offset, int length)

{

    var result = string.Empty;

    if (buffer != null && buffer.Length > 0 && length > 0)

    {

        result = Encoding.UTF8.GetString(buffer, offset, length);

    }

    return result;

}

When a null string is inserted with the Encode method the decodeString method used by Decode transform it in emptyString.

So the Encode / Decode is not idempotent for the strings

maybe is better to use string result= null instead of var result = string.Empty;