fix: incorrect warning is shown when build for iOS
fix: incorrect warning is shown when build for iOS

When you do not have iOS native source code in `<path to App_Resources>/iOS/src`, CLI shows warning that there's no modulemap and you'll not be able to use the code through JavaScript. However, there's no code at all. Skip the preparation of native source code in case there's no native sources there.

NativeScript/nativescript-cli,NativeScript/nativescript-cli,NativeScript/nativescript-cli,NativeScript/nativescript-cli