Fabric component: codegen drops optional state for component properties and in events properties

Description

When adding optional values to the specs for a Fabric component, these optionals are always bridged with a default value and as a result are always defined and no longer optional.

Example spec:

type Prop1 = Readonly<{
    propContent: string;
    propOptionalContent1?: string;
    propOptionalContent2?: string;
}>;
type Prop2 = Readonly<{
    propContent: string;
    propOptionalContent1?: string;
    propOptionalContent2?: string;
}>;
export type NativeEvent1 = Readonly<{
    nativeEventContent: string;
    nativeEventOptionalContent1?: string;
    nativeEventOptionalContent2?: string;
}>;
export type NativeEvent2 = Readonly<{
    nativeEventContent: string;
    nativeEventOptionalContent1?: string;
    nativeEventOptionalContent2?: string;
}>;
export interface NewArchViewProps extends ViewProps {
    prop1: Prop1;
    prop2: Prop2;
    onNativeEvent1: DirectEventHandler<NativeEvent1>;
    onNativeEvent2: DirectEventHandler<NativeEvent2>;
}

Codegen creates:

struct NewArchViewProp1Struct {
  std::string propContent{};
  std::string propOptionalContent1{};
  std::string propOptionalContent2{};
};
struct NewArchViewProp2Struct {
  std::string propContent{};
  std::string propOptionalContent1{};
  std::string propOptionalContent2{};
};
struct OnNativeEvent1 {
  std::string nativeEventContent;
  std::string nativeEventOptionalContent1;
  std::string nativeEventOptionalContent2;
};
struct OnNativeEvent2 {
  std::string nativeEventContent;
  std::string nativeEventOptionalContent1;
  std::string nativeEventOptionalContent2;
};

Every time an optional value is left out on 1 side of the bridge, the other side receives a default value (e.g optional string is filled in with empty string ""). Setting a default using WithDefault<> gives control over this default, but still does not respect the optional status of the properties.

As a result the old architecture versus the new architecture result in completely different objects being bridged. One has optional data where the other is always prop complete but with defaults.

Steps to reproduce

Add the above specs to a new arch enabled project and check the generated props.h and EventEmmiter.h for the generated code.

The linked reproducer has these in place.

React Native Version

0.76.7

Affected Platforms

Runtime - iOS

Areas

Codegen

Output of npx @react-native-community/cli info

System:
  OS: macOS 15.0.1
  CPU: (8) arm64 Apple M1 Pro
  Memory: 298.75 MB / 32.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 22.11.0
    path: ~/.nvm/versions/node/v22.11.0/bin/node
  Yarn:
    version: 1.22.22
    path: ~/.nvm/versions/node/v22.11.0/bin/yarn
  npm:
    version: 10.9.0
    path: ~/.nvm/versions/node/v22.11.0/bin/npm
  Watchman:
    version: 2024.08.19.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.16.2
    path: /opt/homebrew/bin/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 24.2
      - iOS 18.2
      - macOS 15.2
      - tvOS 18.2
      - visionOS 2.2
      - watchOS 11.2
  Android SDK: Not Found
IDEs:
  Android Studio: 2024.1 AI-241.18034.62.2411.12169540
  Xcode:
    version: 16.2/16C5032a
    path: /usr/bin/xcodebuild
Languages:
  Java:
    version: 17.0.14
    path: /Library/Java/JavaVirtualMachines/zulu-17.jdk/Contents/Home/bin/javac
  Ruby:
    version: 3.3.5
    path: /opt/homebrew/opt/ruby/bin/ruby
npmPackages:
  "@react-native-community/cli":
    installed: 15.0.1
    wanted: 15.0.1
  react:
    installed: 18.3.1
    wanted: 18.3.1
  react-native:
    installed: 0.76.5
    wanted: 0.76.5
  react-native-macos: Not Found
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: true
  newArchEnabled: true

Stacktrace or Logs

Example logs from the reproducer (reproducer/no-optionals branch) showing empty strings being bridged instead of remaining optional:

'nativeEvent1 optionalContent1 received:', '76BF877F-B476-4DB6-B961-C0DDA76B91E1'
'nativeEvent1 optionalContent2 received:', '24E3266D-7451-40E4-8A38-B5931A9ACDD1'
'nativeEvent2 optionalContent1 received:', ''
'nativeEvent2 optionalContent2 received:', ''
propOptionalContent1: optional1
propOptionalContent2: optional2
propOptionalContent1:
propOptionalContent2:

Reproducer

https://github.com/wvanhaevre/NewArchReproducer/tree/reproducer/no-optionals

Screenshots and Videos

No response

Start with the TypeScript component and event specs from the issue, then reproduce the output in generated props.h and EventEmmiter.h using the linked reproducer's reproducer/no-optionals branch. Trace the codegen handling of optional properties and compare generated props and events with the example. Done means omitted optional values remain optional rather than becoming default-initialized values on the other side of the bridge.