Fix datagrid not updating when not attached to tree

see #9527

src/Avalonia.Controls.DataGrid/DataGrid.cs

@@ -2101,6 +2101,7 @@ namespace Avalonia.Controls
             if (DataConnection.DataSource != null && !DataConnection.EventsWired)
             {
                 DataConnection.WireEvents(DataConnection.DataSource);
+                InitializeElements(false /*recycleRows*/);
             }
         }