Add tests showing collection callback is called with undefined when setting null on a collection key

tests/unit/onyxTest.ts

@@ -1077,6 +1077,37 @@ describe('Onyx', () => {
         );
     });
 
+    it('should not call the callback with undefined to a collection subscriber using waitForCollectionCallback when deleting one of the collection keys', () => {
+        const mockCallback = jest.fn();
+
+        // Given an Onyx.connect call with waitForCollectionCallback=true
+        connection = Onyx.connect({
+            key: ONYX_KEYS.COLLECTION.TEST_POLICY,
+            waitForCollectionCallback: true,
+            callback: mockCallback,
+        });
+        Onyx.set(`${ONYX_KEYS.COLLECTION.TEST_POLICY}${1}`, {someData: 1});
+        return (
+            waitForPromisesToResolve()
+                // When mergeCollection is called with an updated collection
+                .then(() => Onyx.set(`${ONYX_KEYS.COLLECTION.TEST_POLICY}${1}`, null))
+                .then(() => {
+                    // TODO: not sure why 3 times, I see it gets called with:
+                    // - { testPolicy_1: { someData: 1 } } testPolicy_ (expected)
+                    // - { testPolicy_1: { someData: 1 } } undefined (not expected)
+                    // - { testPolicy_1: undefined } testPolicy_ (this is showing what I think is a problem)
+                    expect(mockCallback).toHaveBeenCalledTimes(3);
+
+                    // We should not call the callback with {testPolicy_1: undefined}
+                    expect(mockCallback).toHaveBeenLastCalledWith({}, ONYX_KEYS.COLLECTION.TEST_POLICY);
+
+                    // The call above should fail, but for some reason it is not, seems like jest is treating
+                    // {testPolicy_1: undefined} the same as {} so adding this check that shows the failure
+                    expect(Object.keys(mockCallback.mock.calls[mockCallback.mock.calls.length - 1][0])).toBe([]);
+                })
+        );
+    });
+
     it('should return a promise when set() called with the same value and there is no change', () => {
         const promiseOne = Onyx.set('test', 'pizza');
         expect(promiseOne).toBeInstanceOf(Promise);