Remove CustomUpdateData heap allocation in OnUpdate hot path.

[stonstad]

This change removes heap allocation of CustomUpdateData in the OnUpdate hot path when custom data does not exist. A read-only CustomUpdateData.Empty property is added to reuse a default instance and remove a per-update allocation.

[DarthAffe]

Ah, that's a nice catch - my first thought seeing this was, that there are actually no places where this is relevant, but the DeviceUpdateTriggers call this.

Just a small suggestion to get around warnings regarding the naming convention.