Handle null params in the Interop TM layer (#49873)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/49873

In the old architecture, when we were passing a `null` value as a parameter in a function that accepted nullable parameter, the null value was mapped to `nil` on iOS.

After my changes in [d4236791e2](https://github.com/facebook/react-native/commit/d4236791e238a614d2fadf5c5659874d983ab029), in the New Architecture, through the interop layer, legacy modules were receiving an `NSNull` object instead of nil.

This was breaking those modules which started crashing or observing undesired behavior.

This change fixes the issue by making sure that, in those cases, a `nil` value is passed.

Note that nested objects in the old architecture were correctly receiving NSNull, so nested objects were behaving correctly already.

## Changelog:
[iOS][Fixed] - Properly pass `nil` for nullable parameters instead of `NSNull` for legacy modules

Reviewed By: javache

Differential Revision: D70723460

fbshipit-source-id: 384f48b6dbb3f54c369b31b6d2ee06069fa3591c