Inconsistency between `merge` & `mergeCollection` when merging a deeply nested `null` value

There are 2 ways to merge a specific item that belongs to a collection:

Calling Onyx.merge() directly;
Calling Onyx.mergeCollection() for the collection holding this item.

These 2 operations must be interchangeable, meaning the same result should be obtained by calling either operation. However, they handle a deeply nested null differently: merge removes the value completely, while mergeCollection sets the value to null and returns it to the subscriber.

Here's the Unit test for the same operation called via merge and mergeCollection:

Expand for code

```js describe('merge', () => { it('should remove a deeply nested null when merging an existing key', () => { let result; const routineRoute = `${ONYX_KEYS.COLLECTION.ROUTES}routine`; connectionID = Onyx.connect({ key: routineRoute, initWithStoredValues: false, callback: (value) => result = value, }); const initialValue = { waypoints: { 1: 'Home', 2: 'Work', 3: 'Gym', }, }; return Onyx.set(routineRoute, initialValue) .then(() => { expect(result).toEqual(initialValue); Onyx.merge(routineRoute, { waypoints: { 1: 'Home', 2: 'Work', 3: null, }, }); return waitForPromisesToResolve(); }) .then(() => { expect(result).toEqual({ waypoints: { 1: 'Home', 2: 'Work', } }); }); }); }); describe('mergeCollection', () => { it('should remove a deeply nested null when merging an existing collection item', () => { let result; const routineRoute = `${ONYX_KEYS.COLLECTION.ROUTES}routine`; connectionID = Onyx.connect({ key: ONYX_KEYS.COLLECTION.ROUTES, initWithStoredValues: false, waitForCollectionCallback: true, callback: (value) => result = value, }); const initialValue = { waypoints: { 1: 'Home', 2: 'Work', 3: 'Gym', }, }; return Onyx.set(routineRoute, initialValue) .then(() => { expect(result).toEqual({[routineRoute]: initialValue}); Onyx.mergeCollection(ONYX_KEYS.COLLECTION.ROUTES, { [routineRoute]: { waypoints: { 1: 'Home', 2: 'Work', 3: null, }, } }); return waitForPromisesToResolve(); }) .then(() => { expect(result).toEqual({ [routineRoute]: { waypoints: { 1: 'Home', 2: 'Work', }, }, }); }); }); }); ```

https://github.com/paultsimura/react-native-onyx/blob/8c0f4f2e38e76e7b4193b525869ab32a92c6c4ee/tests/unit/onyxTest.js#L1049-L1137

The merge test passes, while the mergeCollection test fails because the actual result is:

{
  "routes_routine": {
    "waypoints": {
      "1": "Home",
      "2": "Work",
      "3": null
    }
  }
}

Expensify / react-native-onyx

Inconsistency between `merge` & `mergeCollection` when merging a deeply nested `null` value #516