generic.NewResource should add zero value

[delaneyj]

When creating sys.timeRes = generic.NewResource[Time](w) I'd expect the zero value to be present, but its a nil value causing a panic. Maybe should be an upsert of zero value if resource doesn't already exist?

[mlange-42]

As resources can be added and removed dynamically, IMO it makes sense to indicate absence by nil.

[delaneyj]

maybe then generic.UpsertResource[Time](w)?