add check for JsonModel to SmartyStrategy

This fixes an issue with returning `JsonModel`s from actions.

Without the check, the following will not work:
```php
public function indexAction() {
    return new JsonModel();
}
```
The above code will lead to an Exception, as the ViewResolver cannot find a template for the action.

The `SmartyStrategy` should not inject the `SmartyRenderer` if the model is a `JsonModel`, but instead do simply nothing and let the `JsonRenderer` of ZF take over. Also it should do nothing if there is no `ViewModel` at all.