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

include/wx/stc/stc.h

@@ -5239,7 +5239,7 @@ public:
 
     virtual void Remove(long from, long to) wxOVERRIDE
     {
-        Replace(from, to, "");
+        Replace(from, to, wxString());
     }
     virtual void Replace(long from, long to, const wxString& text) wxOVERRIDE
     {