Apply the rest of the changes in #15586.  Closes #15586.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxPython/trunk@75433 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Since a reference is being returned it is not a good idea to return a new wxString() or wxEmptyString (which is “”) so just give it a private member to be returned instead.  These stubs should never actually be called so it doesn’t really matter much, but let’s keep the compiler happy.  Closes #15491.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxPython/trunk@75434 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
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git-svn-id: https://svn.wxwidgets.org/svn/wx/wxPython/trunk@75435 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
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git-svn-id: https://svn.wxwidgets.org/svn/wx/wxPython/trunk@75436 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
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git-svn-id: https://svn.wxwidgets.org/svn/wx/wxPython/trunk@75437 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775