fix: skip hook when code assets are disabled

[HeyChengdu]

The code_assets API documents HookConfig.code as available only when buildCodeAssets is true. Flutter 3.44.6 can invoke this build hook during flutter run without requesting Code Assets, so the unconditional access currently throws from CodeConfig._fromJson after the macOS app has compiled and launched.

This guard makes that unsupported hook invocation a successful no-op while preserving the existing Rust build path whenever Code Assets are requested.

Reproduction before the fix:

flutter run -d macos
✓ Built build/macos/Build/Products/Debug/SkillsGo.app
Null check operator used on a null value
CodeConfig._fromJson
Building assets for package:velopack_flutter failed.

Verified after the fix:

• flutter run -d macos reaches the Dart VM Service and restores hot reload
• flutter analyze passes in this package