[router] Prevent crash when EXPO_ROUTER_APP_ROOT is missing

[marklawlor]

Why

require-context-ponyfill crashes when EXPO_ROUTER_APP_ROOT is missing. This module is used by the Typed Routes feature

How

Ensure the directory exists before we attempt to read it

Test Plan

Created unit tests that assert against the current filesystem without mocks

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for npx expo prebuild & EAS Build (eg: updated a module plugin).

[EvanBacon]

Add an E2E expo customize test in a project with no routes to ensure this doesn't regress.