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/common/docview.cpp

@@ -1124,7 +1124,7 @@ void wxDocManager::OnFileNew(wxCommandEvent& WXUNUSED(event))
 
 void wxDocManager::OnFileOpen(wxCommandEvent& WXUNUSED(event))
 {
-    if ( !CreateDocument("") )
+    if ( !CreateDocument(wxString()) )
     {
         OnOpenFileFailure();
     }