Fix using const char* and wxString in ternary operator

Using a const char* and a wxString in the two branches of the ternary
operator resulted in compile-time errors since the result type of the
conditional expression was ambiguous, so add explicit conversions to fix
it.

Closes https://github.com/wxWidgets/wxWidgets/pull/2634

See #19355.

samples/widgets/dirctrl.cpp

@@ -275,7 +275,7 @@ void DirCtrlWidgetsPage::CreateDirCtrl(bool defaultPath)
     wxGenericDirCtrl *dirCtrl = new wxGenericDirCtrl(
         this,
         DirCtrlPage_Ctrl,
-        defaultPath ? wxDirDialogDefaultFolderStr : m_dirCtrl->GetPath(),
+        defaultPath ? wxString(wxDirDialogDefaultFolderStr) : m_dirCtrl->GetPath(),
         wxDefaultPosition,
         wxDefaultSize,
         style