Small cleanups for IntersectionObserver (#38099)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/38099

A couple of small cleanups after the bulk of the work for `IntersectionObserver` was already merged.

Changelog: [internal]

Reviewed By: NickGerleman

Differential Revision: D47056943

fbshipit-source-id: 2a6f9062d91da458f8b980f3abcdc9a4ac86389b

packages/react-native/Libraries/IntersectionObserver/__mocks__/NativeIntersectionObserver.js

@@ -97,8 +97,12 @@ const NativeIntersectionObserverMock = {
     );
     observations.splice(observationIndex, 1);
   },
-  connect: (notifyIntersectionObserversCallback?: () => void): void => {
+  connect: (notifyIntersectionObserversCallback: () => void): void => {
     invariant(callback == null, 'unexpected call to connect');
+    invariant(
+      notifyIntersectionObserversCallback != null,
+      'unexpected null notify intersection observers callback',
+    );
     callback = notifyIntersectionObserversCallback;
   },
   disconnect: (): void => {