test+docs: improve tests and examples about allowNull

[vkarpov15]

Summary

Add a quick example and some tests elaborating on how allowNull handles keys that are explicitly set to undefined.

Examples

[Copilot]

Pull request overview

This PR improves coverage and documentation around how allowNull: false behaves when paths are omitted vs explicitly set to undefined, including behavior in validation, saving, and findOneAndUpdate() with runValidators.

Changes:

• Expand validation tests to cover {} vs { name: undefined } for allowNull: false.
• Adjust findOneAndUpdate() tests to demonstrate that $set: { name: undefined } is stripped and does not modify the stored value.
• Update SchemaType#allowNull JSDoc example to include the {} case and clarify undefined behavior.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
test/schema.validation.test.js	Adds a validation case for an omitted allowNull: false path ({}) to confirm it isn’t implicitly required.
test/model.findOneAndUpdate.test.js	Updates the runValidators test to assert behavior when $set contains undefined (no-op after Mongoose strips it).
test/document.test.js	Adds a save/insert regression test illustrating how undefined is handled for allowNull: false paths (top-level and nested).
lib/schemaType.js	Updates the allowNull API docs example to cover omitted paths and clarify undefined treatment.

[AbdelrahmanHafez]

One comment, otherwise LGTM

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.