fix(model+document): avoid depopulating manually populated doc as getter value

[vkarpov15]

Fix #14759

Summary

When depopulating, it looks like Mongoose applies _id getters, which is problematic if you add a default getter to convert all ObjectIds to strings like here: https://thecodebarbarian.com/whats-new-in-mongoose-54-global-schematype-configuration.html#schematype-getters. Fixed by replacing doc._id with doc._doc._id to get the raw id and bypass any getters.

We should backport this to 7.x and 6.x. Same issue impacts those versions.

Examples

[hasezoey]

LGTM, minor style suggestion and would like to see the question above resolved