LightningProperties's priority_rules and exemption_rules are not marked optional

[Quezler]

https://lua-api.factorio.com/latest/typ ... tion_rules

if you set/keep them at nil the game loads up just fine, this leads modders into a false sense of security that they don't have to check if the array exists first.

so it would be nice if they were both marked as optional, or just forced to at least be empty arrays.