[native_assets_cli] `input.config.code.android.target_ndk_api` should be required in JSON

[dcharkes]

The current way the code is set up, a missing input.config.code.android.target_ndk_api if input.config.code.android is provided is treated as a semantic error.

This should be a syntactic error. The JSON schema should describe this field as required, so accessing this field should be non-nullable in the generated Dart parser and lead to a syntactic error. Otherwise, we'd need to treat everything that's non-nullable in the syntax as nullable, and make these semantic errors, this would not take of advantage that we have a schema to describe the syntax.

Filing a separate issue so that we can keep the PR introducing the schemas (#95) a non-op for the rest of the implementation.

A list of fields which have this issue:

• HookInput.config.code.android.target_ndk_api
• HookInput.config.code.ios.target_sdk
• HookInput.config.code.ios.target_version
• HookInput.config.code.macos.target_version
• CodeAsset.architecture