fix(populate): handle deselecting _id with array of fields in populate()

[vkarpov15]

Fix #14231

Summary

We handle deselecting _id in populate() using the following syntax: await U.findOne({ name: 'u1' }).populate('comments', '-_id'); but putting -_id in an array await U.findOne({ name: 'u1' }).populate('comments', ['-_id']); leads to a malformed result document where comments is [ { '0': '-_id' } ] due to how we shallow copy select conditions before removing _id. So we need an additional case to avoid using { ... } to shallow copy an array.

Examples