typescript: Allow null for optional document fields

[JavaScriptBach]

Summary

When inferring types, allow null as well as undefined if the field is optional.

Fixes #12748

Examples

[Uzlopak]

Accidently closed.

Null has a special meaning in mongo. I doubt if this is a desired behavior.

[JavaScriptBach]

We can insert null into a Mongo collection, so why should that not be allowed for optional Mongoose fields?

[Uzlopak]

Nobody says that null is not a valid input. But while JSON.stringify removes keys with value null, the bson serializer serializes null values.
So the expectations can be different.

So we should keep this in mind when merging this.

[JavaScriptBach]

I think it is the opposite actually. JSON.stringify() removes undefined keys, but keeps null keys: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify#description

[vkarpov15]

This works, thank you for making this change. You are correct that you can set any non-required property to null.

My only concern is that this change might be a bit heavy for a patch release, so we'll save it for our next minor release.

[JavaScriptBach]

It turns out what I really need for my use case is for optional strings to infer as foo: string | null | undefined instead of foo ?: string | undefined. But that seems a different direction than what was taken in this project, and I don't suppose you're willing to change the inference to a required field unioned with null and undefined?

So feel free to merge or not merge this PR, but I no longer have a stake in this issue now. Thanks for reviewing either way.

[vkarpov15]

Merging into 6.9