fix(document): allow setting nested path to null

[vkarpov15]

Fix #14205

Summary

Still debugging the need for the valForKey === null check, but otherwise this works. Without the === null check, the with conflicted data in db test in model.test.js fails; it looks like because we don't store validation errors on nested paths. Will take another look tomorrow.

Examples

[AbdelrahmanHafez]

LGTM, thanks 👍

[vkarpov15]

I took a closer look and the == null check is necessary because of this code later on in $set which handles setting a nested path to non-nullish.