Not sure about how Spark Wallet handles this. Using C-lightning RPC interface you can avoid specifying optional parameters, just by not specifying them.

For your usage question, please refer to [the manual][1]. It specifies that `exclude` must be a JSON array and even gives you an example:
```
[“564334x877x1/0”,“564195x1292x0/1”]
```

  [1]: https://lightning.readthedocs.io/lightning-getroute.7.html