v6: populate makes single document query type non-null

[aaronadamsCA]

All of these should have the same type, but they do not:

import { model } from "mongoose";

const MyModel = model("MyModel");

let maybeDoc = await MyModel.findById(null).exec();
typeof maybeDoc; // (Document<any, any, unknown> & { _id: Types.ObjectId }) | null

maybeDoc = await MyModel.findById(null).populate("someField").exec();
typeof maybeDoc; // Document<any, any, unknown> & { _id: Types.ObjectId }

maybeDoc = await MyModel.findOne({}).exec();
typeof maybeDoc; // (Document<any, any, unknown> & { _id: Types.ObjectId }) | null

maybeDoc = await MyModel.findOne({}).populate("someField").exec();
typeof maybeDoc; // Document<any, any, unknown> & { _id: Types.ObjectId }

This worked as expected in Mongoose 5.13.13. During migration, ESLint flagged some "unnecessary" null checks.

Node.js 16.13.1, Mongoose 6.0.14.

[IslandRhythms]

_id is unknown for me and there is no | null.

[aaronadamsCA]

I've provided a reproduction here:

https://codesandbox.io/s/mongoose-issue-11041-q74uu?file=/index.ts