fix: set correct optionality when decoding list type

[dpilch]

Description of changes

Set correct optionality when decoding list types.

List types have four permutation when it comes to optionality.

1. Required list, required element [String!]!
2. Required list, nullable element [String]!
3. Nullable list, required element [String!]
4. Nullable list, nullable element [String]

The decoding is currently invalid for some of these cases.

---

The decoding should be:

1. Required list, required element [String!]!

myCustomTypes = try values.decode([MyCustomType].self, forKey: .myCustomTypes)

2. Required list, nullable element [String]!

myCustomTypes = try values.decode([MyCustomType?].self, forKey: .myCustomTypes)

3. Nullable list, required element [String!]

myCustomTypes = try? values.decode([MyCustomType].self, forKey: .myCustomTypes)

5. Nullable list, nullable element [String]

myCustomTypes = try? values.decode([MyCustomType?].self, forKey: .myCustomTypes)

Codegen Paramaters Changed or Added

N/A

Issue #, if available

#885

Description of how you validated changes

• Unit tests

Checklist

• PR description included
• yarn test passes
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)
• 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
• Changes are tested on windows. Some Node functions (such as path) behave differently on windows.
• 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.