title: Hide multibinding stringformat when binding returns null
body: <p>I was looking to bind a timespan property to a textblock,which seems to be resolved with
   the help of <a href="https://stackoverflow.com/q/4563081/217880">this</a> post </p>
  
  <p>Now i want to hide StringFormat when data is null.From the thread if i use a mutibinding with a stringformat and if my data is null then the stringformat displays just a ":"</p>
  
  <pre><code>&lt;TextBlock&gt;
      &lt;TextBlock.Text&gt;
          &lt;MultiBinding StringFormat="{}{0}:{1}"&gt;
              &lt;Binding Path="MyTime.Hours" TargetNullValue={x:Null}/&gt;
              &lt;Binding Path="MyTime.Minutes" TargetNullValue={x:Null}/&gt;
          &lt;/MultiBinding&gt;
      &lt;/TextBlock.Text&gt;
  &lt;/TextBlock&gt;
  </code></pre>
  
  <p>How could i hide the ":" if data is null </p>