[ifmelate]

Fixes found by running generated code against public GraphQL APIs (Countries, Rick and Morty, GitLab, GitHub).

• Variables with a default value are sent with that default when not given. The normalized cache doesn't apply defaults, so @include/@skip on such a variable made the query fail with PartialDataException
• Nullable input list fields default to null instead of [] (both result type modes): [] in a filter means "none of these"
• A schema type named Duration no longer hides dart:core Duration (pollInterval)

Tests: test/public_api_regressions_test.dart, updated runtime checks in the compile test.