title: null string deserialized to empty string when using protobuf-net
body: <p>I am using protobuf-net to serialize and deserialize my messages. My message also contain come strings that can be null. However when I deserialize them on the other side, I get empty strings ("").</p>
  
  <p>According to google docs, the default value for string type in empty string.
  What could be the resolution for this issue?</p>
  
  <p>Here is the code I am using:</p>
  
  <pre><code>Command message = new Command();
  message.s_value = null;
  using (MemoryStream stream = new MemoryStream())
  {
       Serializer.Serialize&lt;Command&gt;(stream, message);                
       stream.Close();
  }
  </code></pre>
  
  <p>Upon deserializing the same stream, I get <code>s_value = ""</code></p>