Parameters.txt: Don't attempt to import model if path is "NONE"

[Gotolei]

On a particular three-wheeled car, the front two wheels are in the same place. Because the wheel only needs to be shown once, the corresponding model for one of them is set to "NONE" in parameters.txt. When the game sees this, it doesn't cause any problems, as the game ignores it. However, it does cause an import fail with the plugin, as it evidently tries to parse it.