Define unambiguous Is{List,Text}Empty() in wxOwnerDrawnComboBox.

Like in wxComboBox itself, it's useful to provide these methods to avoid
problems with ambiguous IsEmpty(), inherited from both wxTextEntry and
wxItemContainer.

Closes #16618.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78015 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775