title: Add to string if string non empty
body: <p>Sometime I want to join two strings with a space in between. But if second string is null, I don't want the space.</p>
  
  <p>Consider following code:</p>
  
  <pre><code>void AssertFoo(bool cond, string message = null) {
      ...
      Assert.Fail("Something is foo.{0}", message != null ? " " + message : "");
      ...
  }
  </code></pre>
  
  <p>Is there a more elegant way to do that?</p>