Fix `WikiHeader` setting `string.Empty` rather than `null` on breadcrumb

That's supposed to be fixed already in the base PR, but somehow it
continued to exist here.

osu.Game/Overlays/Wiki/WikiHeader.cs

@@ -38,7 +38,7 @@ private void onWikiPageChange(ValueChangedEvent<APIWikiPage> e)
                 return;
 
             TabControl.Clear();
-            Current.Value = string.Empty;
+            Current.Value = null;
 
             TabControl.AddItem(IndexPageString);