FormState.List is broken for nested lists

[wrenhawth]

Overview

Related to #519, the recent fix #531 fixed the issue for lists a single level deep. However, if there's a FormState.List within a FormState.List, the component crashes when an item is added to the nested list, because initialValue is undefined instead of an empty array.

I believe adding an additional guard of initialValue || before intialValue[index] to line 49 of List.tsx will solve this issue and it seems to work with local testing, but I do not know the package well enough to thoroughly test for unintended side-effects.

Consuming repo

Shopify/quilt

Checklist

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