Don't crash when changing label of label-less wxToggleButton in wxGTK.

Don't crash when changing label of label-less wxToggleButton in wxGTK.

If a button doesn't show any text label, simply don't do anything when
SetLabel() is called instead of replacing the image shown by the button with a
text label as the button doesn't expect this to happen and doing it breaks its
assumptions about the widgets it has and results in assert failures later.

Closes #13693.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@69830 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775