fix: RN 0.76 fabric compatibility

[vonovak]

Summary

edit: this won't be needed after facebook/react-native#47086

As it is, the project won't build with new architecture on RN 0.76 because the ViewManagerDelegate interface has been refactored to Kotlin and the commandId is now a non-nullable String.

This PR changes the override fun receiveCommand signature to use non-nullable String too.

This is backward-compatible because the overridden function is defined in a Java base class and

override fun receiveCommand(root: NestedScrollableHost, commandId: String, args: ReadableArray?)
override fun receiveCommand(root: NestedScrollableHost, commandId: String?, args: ReadableArray?)

are both valid overrides of Java in Kotlin.

The fix (and the assumption)

Changing to a non-nullable String complies with the ViewManagerDelegate.kt interface and should also be future-compatible because if at some point the base class is also Kotlinified this will still work - assuming that the current Java signature public void receiveCommand(@NonNull T root, String commandId, @Nullable ReadableArray args) becomes open ... fun receiveCommand(root: NestedScrollableHost, commandId: String, args: ReadableArray?) (i.e. the string won't be nullable, in line with how it's defined in ViewManagerDelegate.kt).

Test Plan

TODO

What's required for testing (prerequisites)?

What are the steps to reproduce (after prerequisites)?

Compatibility

OS	Implemented
iOS	✅❌
Android	✅❌

Checklist

• I have tested this on a device and a simulator
• I added the documentation in README.md
• I updated the typed files (TS and Flow)