title: Add XElement only if value exists
body: <p>I'm creating an XDocument using Linq-To-XML, like this:</p>
  
  <pre><code>Order order = GetOrder();
  
  XDocument doc = new XDocument(
      new XElement("purchaseOrder",
          new XElement("Name", order.Name),
          new XElement("Address", order.Address),
          new XElement("Shipper", order.Shipper)
      )
  );
  </code></pre>
  
  <p>So sometimes an order will not have a Shipper, it will be null. In that case, I don't want to include the Shipper element at all.</p>
  
  <p>How can I do that inline in my code when creating the doc?</p>