feat(webAPI): Make all lists nullable in OpenAPI schema (#1359)

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## Description

<!--- Describe your changes in detail -->

## Related Issue(s)

- #1355

## Verification

- [ ] **Your** code builds clean without any errors or warnings
- [ ] Manual testing done (required)
- [ ] Relevant automated test added (if you find this hard, leave it and
we'll help out)

## Documentation

- [ ] Documentation is updated (either in `docs`-directory, Altinnpedia
or a separate linked PR in
[altinn-studio-docs.](https://github.com/Altinn/altinn-studio-docs), if
applicable)


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **New Features**
- Updated API specifications to allow properties to be nullable,
enhancing flexibility in handling optional data.
- Introduced a method to make array-type properties nullable in OpenAPI
document schemas.

- **Bug Fixes**
- Updated serialization logic to include empty collections during data
serialization, affecting the output of serialized data.

- **Tests**
- Enhanced assertion logic in integration tests to require closer
matches for `DateTimeOffset` types.
- Modified dialog retrieval tests to no longer exclude missing members
in assertions, improving accuracy in test validations.
- Refined response handling in dialog search tests for better clarity
and accuracy in expectations.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->