Fix fastMerge not correctly merging all nested null values

[chrispader]

@paultsimura

Details

Previously fastMerge wouldn't recursively call itself when merging an object with a target value of undefined or null. Therefore, nested null values wouldn't be removed either, because this only happens if we use fastMerge for nested keys and not set the value directly.

This PR fixes this issue, renames some variables for more clarity and adds comments to further explain the code.

It also adds 3 more unit tests, that @paultsimura suggested in #301 (comment) and #301 (comment).

Related Issues

#508

Automated Tests

Run the tests in fastMergeTests. I added an additional test that checks for exactly this issue.

Manual Tests

This was reported based on #490, so we might want to test if these changes (partially) fix the issues there.

Author Checklist

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Screenshots/Videos

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

MacOS: Desktop

No screenshots, as this only internally affects Onyx operations.

[paultsimura]

@chrispader thanks for looking into this!
Could you please verify this fix doesn't reintroduce this issue? There was some talk about intentionally keeping nulls in cache during the mergeCollection.

[chrispader]

This PR fixes only the problem in fastMerge, this doesn't fix the inconsistency between Onyx.merge and Onyx.mergeCollection.

Thinking about this, there might also be a discrepancy between Onyx.set and Onyx.multiSet. I'm gonna touch on that in another PR as well.

[chrispader]

@paultsimura i can't reproduce the issue from #408 (comment), so i think this PR should be ready for merge!

[tgolen]

Thank you for the wonderful comments to explain why the logic is doing what it is.

[thienlnam]

Looks great, thanks!