Avoid sending null `session` option with document ops

[vkarpov15]

Prerequisites

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

Mongoose version

6.9.x

Node.js version

16.x

MongoDB server version

Any

Typescript version (if applicable)

No response

Description

'use strict';

const mongoose = require('mongoose');
mongoose.set('debug', true);

run().catch(err => console.log(err));

async function run() {
  await mongoose.connect('mongodb://localhost:27017/test');
  const schema = new mongoose.Schema({ name: String });
  const Test = mongoose.model('Test', schema);

  // Mongoose: tests.insertOne({ name: 'foo', _id: new ObjectId("63f28c9760c00f9e5b3d6001"), __v: 0 }, { session: null })
  await Test.create({ name: 'foo' });
}

Sending a null session is unnecessary and confusing. session shouldn't show up in debug output if you're not using sessions.

Steps to Reproduce

See description

Expected Behavior

No response