Always arrange row/col presenters to final size.

Even when they have no items - otherwise star columns will not be sized correctly, resulting in #209.

Fixes #209.

src/Avalonia.Controls.TreeDataGrid/Primitives/TreeDataGridPresenterBase.cs

@@ -359,7 +359,7 @@ protected override Size MeasureOverride(Size availableSize)
         protected override Size ArrangeOverride(Size finalSize)
         {
             if (_realizedElements is null)
-                return default;
+                return finalSize;
 
             _isInLayout = true;