Missing base locale fallback files for nb and zh in l10n directory

[shraavv]

While attempting to run the app on my local device (Flutter version 3.42.0-0.0.pre) , encountered the following error-

The app was working fine previously, the localization generation now strictly enforces the requirement for base locale fallback files. Since we have locale-specific files like app_nb_NO.arb and app_zh_Hans.arb/app_zh_Hant.arb, Flutter now requires corresponding base files (app_nb.arb and app_zh.arb) to exist as fallbacks.