No real changes, just use wxString::clear() instead of assignment.

Consistently use clear() everywhere instead of assigning wxEmptyString in some
places and wxT("") in some others to a string in order to clear it.

See #15091.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73632 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775