fix: set relational fields on many-to-many join table to optional

[dpilch]

Description of changes

[BREAKING CHANGE]: set relational fields on many-to-many join table to optional

API change: https://github.com/aws-amplify/amplify-category-api/pull/2660

Codegen Paramaters Changed or Added

N/A

Issue #, if available

N/A

Description of how you validated changes

• Unit testing
• E2E testing
• TODO: check impact on runtime libraries

Checklist

• [x] PR description included
• [x] yarn test passes
• [x] Tests are changed or added
• [ ] Relevant documentation is changed or added (and PR referenced)
• [x] Breaking changes to existing customers are released behind a feature flag or major version update
• [ ] Changes are tested using sample applications for all relevant platforms (iOS/android/flutter/Javascript) that use the feature added/modified
• [x] Changes adhere to the GraphQL Spec and supports the GraphQL types type, input, enum, interface, union and scalar types.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.