Fix bug in tree view style.

ItemsPresenter didn't have an ID. Also don't return null for logical
children of an ItemsControl with no presenter.

Perspex.Controls/ItemsControl.cs

@@ -96,6 +96,12 @@ namespace Perspex.Controls
                     panel.VisualChildren,
                     x => (ILogical)x);
             }
+            else
+            {
+                this.logicalChildren = new PerspexReadOnlyListView<IVisual, ILogical>(
+                    new PerspexList<IVisual>(),
+                    x => (ILogical)x);
+            }
         }
 
         private void ItemsChanged(Tuple<IEnumerable, IEnumerable> value)

Perspex.Themes.Default/TreeViewItemStyle.cs

@@ -101,6 +101,7 @@ namespace Perspex.Themes.Default
                     },
                     new ItemsPresenter
                     {
+                        Id = "itemsPresenter",
                         Margin = new Thickness(24, 0, 0, 0),
                         [~ItemsPresenter.ItemsProperty] = control[~TreeViewItem.ItemsProperty],
                         [~ItemsPresenter.ItemsPanelProperty] = control[~TreeViewItem.ItemsPanelProperty],