$inc on non-existing field results in NaN

[Hybrid-Force]

Prerequisites

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

Mongoose version

6.6.1

Node.js version

16.15.0

MongoDB server version

4.2.1

Description

According to MongoDB document on $inc,

If the field does not exist, $inc creates the field and sets the field to the specified value.

In Mongoose, $inc operations on non-existing fields result in correct data in the database, however, the field in Mongoose document will become NaN because currentValue is undefined.

mongoose/lib/document.js

Lines 1738 to 1740 in 586c86f

	const currentValue = this.$__getValue(path);
	this.$__setValue(path, currentValue + val);

Steps to Reproduce

import mongoose from 'mongoose';
const schema = new mongoose.Schema({
    count: Number,
    // no default value, not required
});
const Model = mongoose.model('IncTest', schema);
await Model.collection.drop();
const doc = new Model({
    // optional 'count' field no set
});
await doc.save();
doc.$inc('count', 1);
await doc.save();

// doc.count is expected to be 1.
// But instead, it is NaN

Expected Behavior

No response