Fix scripting bug where missing parameters gave syntax error

In these cases the default parameters should have been used instead.
Also the Help command now gives Help on Help by default.

src/Shuttle.cpp

@@ -245,6 +245,8 @@ bool Shuttle::TransferString( const wxString & Name, wxString & strValue, const
       {
          strValue = mValueString;
       }
+      else
+         return false;
    }
    else
    {