Use the bindingcontext of listview if item data is null

Use the binding context of the listview parent control in cases where recycling a view's data is null.  This is likely going to be the global header/footer and possibly section header/footer too

Author: Redth

VirtualListView/Controls/VirtualListView.cs

@@ -322,7 +322,12 @@ public void RecycleView(PositionInfo position, object data, IView view)
 	{
 		if (view is View controlsView)
 		{
-			controlsView.SetValue(View.BindingContextProperty, data);
+			// Use the listview's binding context if
+			// there's no data
+			// this may happen for global header/footer for instance
+			controlsView.SetValue(
+				View.BindingContextProperty,
+				data ?? this.BindingContext);
 		}
 	}