having a null no_records_message should short-circuit the display, just not output a message

Without this, setting:
```
$view->no_records_message = null;
$view->display();
```

will still output `<table>` tags, table-rows, etc. when you would expect it to just not show anything. If this is the expected behavior and I'm just wrong, I could update my code to check if there are 0 rows and set the visibility of the view.