Use wxString() instead of "" for empty strings

This will allow this code to work even when implicit conversion from
"const char*" is disabled in wxString and is already marginally more
efficient even now.

See https://github.com/wxWidgets/wxWidgets/pull/782

src/xrc/xmlres.cpp

@@ -2584,7 +2584,7 @@ void wxXmlResource::ReportError(const wxXmlNode *context, const wxString& messag
 {
     if ( !context )
     {
-        DoReportError("", NULL, message);
+        DoReportError(wxString(), NULL, message);
         return;
     }