Crash: Unexpectedly found nil while unwrapping an Optional value

[jcmosc]

I've attached a PR that adds a new test case to reproduce a crash when nesting multi root variadic views:
#790

OpenSwiftUICore/ForEach.swift:709: Fatal error: Unexpectedly found nil while unwrapping an Optional value

The nil value is passed into ViewList.applySublists() by VariadicView.Children.Element. Later ForEachState.applyNodes force unwraps the list parameter in various places on both the static and dynamic code paths. Though I can only reproduce this on the dynamic code path.

I'm not sure what the fix should be, i.e. whether a non-nil value should be passed in, or if ForEachState should expect nil values sometimes. Hopefully this repro helps you solve it.

[Kyle-Ye]

I will review and address this after the Chinese New Year, as I will be away from the internet for a few days~