Fix assert when adding items with bitmaps wxBitmapComboBox.

The change of r73567 resulted in an assert when adding items with bitmaps to
wxBitmapComboBox without object client data. Fix the code added by this commit
to work for all kinds of client data, including wxClientData_None.

See #14892.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73880 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775