Change runDataActiveRun replicant empty state to null instead of undefined

[zoton2]

Right now, the runDataActiveRun replicant having no run set is indicated by it being undefined. The issue is the generated type doesn't represent that, and the JSON schema is a bit unclear although it does work. I was thinking about doing this before but didn't, and not sure why. Maybe null isn't the right setting for a run not being set, but in NodeCG's case it's easier to work with. This would obviously be a breaking change so would be targeting a v3.x.x if I decided to go ahead with it.