title: WPF: Fire data trigger when the value of the string is null or string.Empty
body: <p>I have created a <code>DataTemplate</code></p>
  
  <pre><code>&lt;DataTemplate DataType="{x:Type model:Person}"&gt;
    &lt;StackPanel&gt;
      &lt;!-- Text box is binding to the person's Image property. --&gt;
      &lt;TextBlock Text="{Binding Image}" /&gt;
      &lt;Border&gt;
        &lt;Border.Style&gt;
          &lt;Style TargetType="Border"&gt;
            &lt;Style.Triggers&gt;
              &lt;!-- Data trigger is binding to the same Image property. --&gt;
              &lt;DataTrigger Binding="{Binding Image}" Value="{x:Null}"&gt;
                &lt;Setter Property="Background"&gt;
                  &lt;Setter.Value&gt;
                    &lt;LinearGradientBrush StartPoint="0,0" EndPoint="0,1"&gt;
                      &lt;GradientStop Color="#696969" Offset="0.0" /&gt;
                      &lt;GradientStop Color="#2E2E2E" Offset="1.0" /&gt;
                    &lt;/LinearGradientBrush&gt;
                  &lt;/Setter.Value&gt;
                &lt;/Setter&gt;
              &lt;/DataTrigger&gt;
            &lt;/Style.Triggers&gt;
          &lt;/Style&gt;
        &lt;/Border.Style&gt;
      &lt;/Border&gt;
    &lt;/StackPanel&gt;
  &lt;/DataTemplate&gt;
  </code></pre>
  
  <p>The <code>Person</code> class looks like this: </p>
  
  <pre><code>public class Person
  {
      public string Image { get; set; }
  }
  </code></pre>
  
  <p>The presented data trigger in the data template does not work as expected. No matter if I provide the value for the <code>Image</code> property or not, it simply won't draw the linear gradient brush background. On the other hand, the text block is showing the content of the <code>Image</code> property as it should. </p>
  
  <p>I thought that maybe when the <code>person.Image = null;</code>, then the XAML is in fact reading it as  <code>person.Image = string.Empty;</code> but I tried replacing <code>Value="{x:Null}"</code> with <code>Value=""</code> and it still doesn't work. </p>
  
  <p>Help please, thanks. </p>