Empty documents were incorrectly added when lean populating a nested undefined array

[csy1204]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

8.0.1

Node.js version

20.9.0

MongoDB server version

5.1

Typescript version (if applicable)

No response

Description

Ref: #8432

If a field in a nested schema is of type array but is an empty field, {} is incorrectly added when doing a lean populate.
An empty array is added just fine in a normal populate, but in a lean populate an empty document is unexpectedly added, making the type inconsistent.
For consistent results, lean populate should be improved to not add it at all.
I want to fix it myself.

Steps to Reproduce

const userSchema = new mongoose.Schema({
  fullName: String,
  company: String
});
const User = db.model('User', userSchema);

const fileSchema = new mongoose.Schema({
  _id: String,
  uploaderId: {
    type: mongoose.ObjectId,
    ref: 'User'
  }
}, { toObject: { virtuals: true }, toJSON: { virtuals: true } });
fileSchema.virtual('uploadedBy', {
  ref: 'User',
  localField: 'uploaderId',
  foreignField: '_id',
  justOne: true
});

const contentSchema = new mongoose.Schema({
  memo: String,
  files: { type: [fileSchema], default: [] }
}, { toObject: { virtuals: true }, toJSON: { virtuals: true }, _id: false });

const postSchema = new mongoose.Schema({
  title: String,
  content: { type: contentSchema }
}, { toObject: { virtuals: true }, toJSON: { virtuals: true } });
const Post = db.model('Test1', postSchema);

const user = await User.create({ fullName: 'John Doe', company: 'GitHub' });
await Post.create([
  { title: 'London-Paris' },
  {
    title: 'Berlin-Moscow',
    content: {
      memo: 'Not Easy',
      files: [{ _id: '123', uploaderId: user._id }]
    }
  }
]);

await Post.updateMany({}, { $unset: { 'content.files': 1 } });

const populatedRides = await Post.find({}).populate({
  path: 'content.files.uploadedBy',
  justOne: true
}).lean();

console.log(populatedRides[0].content);
// { files: { uploadedBy: null } }
console.log(populatedRides[1].content);
// { memo: 'Not Easy', files: { uploadedBy: null } }

As you can see above, files is expected to be an array, but the {} is being added incorrectly, giving us the wrong result altogether.

Expected Behavior

console.log(populatedRides[0].content);
// { }
console.log(populatedRides[1].content);
// { memo: 'Not Easy' }