title: CollectionViewSource.View is null until the first valid source; empty View needed instead
body: <p>My code looks like this:</p>
  
  <pre><code>var telemetry = new CollectionViewSource();
  telemetry.SortDescriptions.Add(new SortDescription("Name", ListSortDirection.Ascending));
  SetTelemetryFilters(telemetry);
  
  var binding = new Binding("Asset.TelemetryDefinitions.Values");
  binding.Source = this;
  var expression = BindingOperations.SetBinding(telemetry, CollectionViewSource.SourceProperty, binding);
  </code></pre>
  
  <p>Shortly after this code I am setting a <code>ComboBox.ItemsSource = telemetry.View</code>. I need view (in multiple places) because it implements <code>IEnumerable</code> and the CollectionViewSource does not. When the software first runs the <code>Asset</code> property is null. This is problematic because that causes telemetry.View to return null, which in turn means my ItemsSource was set to null. Is there some way to make the CollectionViewSource.View return an empty view in the case that the binding operation returns a null to CollectionViewSource.Source? (And I assume that once CollectionViewSource.View returns non-null it always returns the same instance?)</p>