Never collapse invisible root item in generic wxDataViewCtrl.

Never collapse invisible root item in generic wxDataViewCtrl.

Since the changes of r72325, the root item could be collapsed and marked as
not having any children but this was wrong because no items could be added to
it after this.

Just ignore any attempts to collapse it.

Closes #14801.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72905 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775