Protect against inserting documents with null `_id`

For example, the below script (re: #9180) ends up upserting a document with a null `_id`:

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

const { Schema } = mongoose;

(async () => {
const dbUri = 'mongodb://localhost:27017/test';
const db = await mongoose.connect(dbUri, { useNewUrlParser: true });
await db.connection.dropDatabase();

const schema = new Schema({ name: String });
const model = mongoose.model('item', schema);

await model.bulkWrite([{
updateOne: {
filter: { _id: null },
update: { $set: { name: 'John Doe' } },
upsert: true,
},
}]);

console.log(await model.find());

await db.disconnect();
process.exit(0);
})();
```

A document with a null `_id` is technically valid, but many users may see that as a bug. Perhaps we should add some logic to protect against this, at least for Mongoose's default `_id`?

Start by running the JavaScript reproduction in the issue against MongoDB and trace the bulkWrite upsert path it exercises. Identify the existing tests around bulk writes and default _id handling; done should mean the reported case no longer inserts a document with a null _id without breaking valid upserts.

Written by the indexing model from the issue text.