Fix a NullReferenceException in the syntax visualizer.

The syntax visualizer crashed Visual Studio when observing the constant value of `null`. This was because the syntax visualizer was not expecting
GetConstantValue().Value to be null with .HasValue set to true.

src/Tools/Source/SyntaxVisualizer/SyntaxVisualizerControl/SyntaxVisualizerControl.xaml.cs

@@ -778,7 +778,7 @@ private void ConstantValueDetailsMenuItem_Click(object sender, RoutedEventArgs e
                 else
                 {
                     typeTextLabel.Visibility = Visibility.Visible;
-                    typeValueLabel.Content = value.Value.GetType().Name;
+                    typeValueLabel.Content = value.Value?.GetType().Name ?? "<null>";
                     _propertyGrid.SelectedObject = value;
                 }
             }