feat(NODE-7320): Update $ne operator to include null type

[christophehurpeau]

Description

Summary of Changes

Allow $ne operator to accept null values.

null value is useful when we want to have a similar behavior as exists, but including null values even if the Value is not declared as nullable.

Notes for Reviewers

What is the motivation for this change?

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[tadjik1]

Hi @christophehurpeau, thank you for the contribution! We are going to discuss this change internally as there are some usability and necessity questions arose. Can you maybe share the exact use-case or small snippet to demonstrate usage scenario, this would be extremely helpful.

[christophehurpeau]

@tadjik1

Of course.

Most of our models include an schema with non-nullable fields. Here a simple example:

interface EndUser {
  _id: Types.ObjectID;
  fullname: string;
  email?: string;
}

With an index on email.
When we want to query all enduser with existing emails:

const endUsers = await EndUserQueries.find({ email: { $ne: null } })