fix: return nullptr when USE_THIRD_PARTY_JSC is set to true (#49781)

Summary:
When setting `USE_THIRD_PARTY_JSC=1`, we don't return anything from createJSRuntimeFactory function:

![CleanShot 2025-03-03 at 13 05 24@2x](https://github.com/user-attachments/assets/090ca47a-77f8-4a1b-9171-79b5e4ce91fb)

It's meant to be overwritten, but we must satisfy the compiler.

## Changelog:

[IOS] [FIXED] - return nullptr when USE_THIRD_PARTY_JSC is set to true

Pull Request resolved: https://github.com/facebook/react-native/pull/49781

Test Plan: CI Green

Reviewed By: cortinico

Differential Revision: D70486837

Pulled By: cipolleschi

fbshipit-source-id: c09622f75608dc203d41167fc375a000a2f12e37