Handle DocumentArray in FlattenMaps

[Jokero]

Closes #13345

Summary

Since 7.1.0, lean() replaces Map in documents with their Record<string, V> equivalent by using FlattenMaps. Currently, FlattenMaps returns a type incompatible with InferSchemaType type when there is a property of DocumentArray type. This PR fixes it.

Examples

This won't produce compilation error:

import { model, Schema, InferSchemaType } from 'mongoose';
import { expectType } from 'tsd';

async function test() {
    const imageSchema = new Schema({
        url: { required: true, type: String }
    });

    const placeSchema = new Schema({
        images: { required: true, type: [imageSchema] },
    });

    type Place = InferSchemaType<typeof placeSchema>;

    const PlaceModel = model('Place', placeSchema);

    const place = await PlaceModel.findOne().lean().orFail().exec();
    expectAssignable<Place>(place);
}

[vkarpov15]

Thanks 👍