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

Test Plan: CI Green

Reviewed By: cortinico

Differential Revision: D70486837

Pulled By: cipolleschi

fbshipit-source-id: c09622f75608dc203d41167fc375a000a2f12e37

Author: okwasniewski

packages/react-native/Libraries/AppDelegate/RCTDefaultReactNativeFactoryDelegate.mm

@@ -49,6 +49,8 @@ - (JSRuntimeFactoryRef)createJSRuntimeFactory
   return jsrt_create_hermes_factory();
 #elif USE_THIRD_PARTY_JSC != 1
   return jsrt_create_jsc_factory();
+#else
+  return nullptr;
 #endif
 }