title: WPF MultiBinding - UnsetValue Issue
body: <p>I have a TextBlock. When its Text is bound as:</p>
  
  <pre><code>&lt;Binding Path="Applicant2.Surname"/&gt;
  </code></pre>
  
  <p>It works fine, however I want to include the Forenames so changed the binding to:</p>
  
  <pre><code>&lt;MultiBinding StringFormat="{}{0} {1}"&gt;
      &lt;Binding Path="Applicant2.Forenames"/&gt;
      &lt;Binding Path="Applicant2.Surname"/&gt;
  &lt;/MultiBinding&gt;
  </code></pre>
  
  <p>This displays {DependencyProperty.UnsetValue} {DependencyProperty.UnsetValue} until the value is set the first time.</p>
  
  <p>How can I stop this? Why do I not get the problem with the first simple binding?</p>