Check for menu title being non-empty when appending it to the menu bar.

Check for menu title being non-empty when appending it to the menu bar.

It doesn't make sense to add a menu with empty title to the menu bar so assert
that the title is not empty and fail to add the menu if it is.

See r64033 in wxQT branch.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64058 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775