fix: Swift - Generate schema for non-models containing models correctly

[MorFix]

Description of changes

If a schema contains non-models types that contain models, they're not generated correctly in Swift, because of a missing label, and the code doesn't compile.
That scenario can happen if you have a custom resolver that returns a model along with other things.

Issue #, if available

-N/A-

Description of how you validated changes

• Added test
• Generated models locally

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Changes are tested using sample applications for all relevant platforms (iOS/android/flutter/Javascript) that use the feature added/modified

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

[dpilch]

Thanks for the contribution @MorFix. Could you also add this test schema here: https://github.com/aws-amplify/amplify-category-api/blob/main/packages/amplify-graphql-schema-test-library/src/index.ts#L39

This will add this schema to our E2E tests that will test for successful compilation of the generated models.