FBS: LayersChangeNotification body must be optional

[jmillan]

It's none when the spatial layer is -1 internally.

This was crashing the worker when trying to create an empty notification, since the schema was requiring the body.

[nazar-pc]

Layers notification without layers makes little sense to me 🤔

[jmillan]

Yes, same here. But layers are represented by positive numbers. The absence of a positive layer means no layer.

We could make the body mandatory and both fields spatialLayer and temporalLayer optional though.

[ibc]

Yes, same here. But layers are represented by positive numbers. The absence of a positive layer means no layer.

We could make the body mandatory and both fields spatialLayer and temporalLayer optional though.

we emit null to the user anyway so I don't see the point in notifying with undefined/null layers to Node/Rust to make them emit null.