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

Summary:
Pull Request resolved: #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 [d423679](d423679), 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

Author: cipolleschi

packages/react-native/ReactCommon/react/nativemodule/core/platform/ios/ReactCommon/RCTInteropTurboModule.mm

@@ -446,6 +446,15 @@ T RCTConvertTo(SEL selector, id json)
 
     if (objCArgType == @encode(id)) {
       id arg = RCTConvertTo<id>(selector, objCArg);
+
+      // Handle the special case where there is an argument and it must be nil
+      // Without this check, the JS side will receive an object.
+      // See: discussion at
+      // https://github.com/facebook/react-native/pull/49250#issuecomment-2668465893
+      if (arg == [NSNull null]) {
+        return;
+      }
+
       if (arg) {
         [retainedObjectsForInvocation addObject:arg];
       }