fix(dart): require nullable schema properties

[schani]

Summary

• require nullable properties when the schema marks them required
• retain null as an accepted present value

Tests

• npm run build
• npm run lint
• FIXTURE=schema-dart npm run test:fixtures -- test/inputs/schema/strict-optional.schema
• QUICKTEST=true FIXTURE=dart npm run test:fixtures

[github-actions]

Generated-output differences

73 files differ — 73 modified, 0 new, 0 deleted
3060 changed lines — +1530 / −1530

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