feat(schematype): support allowNull option to disallow null values even if not required

[vkarpov15]

Fix #15905

Summary

Mongoose has historically used null and undefined interchangeably, but TypeScript sometimes makes working with potentially null values cumbersome, leading to a lot of ?? undefined. This PR adds a allowNull: false option which allows a value to be undefined, but does not allow said value to be set to null, including automatic type inference in TypeScript.

Examples

[Copilot]

Pull request overview

Adds a new allowNull schema path option to let users disallow null on optional (non-required) paths, with corresponding runtime validation behavior and TypeScript inference updates.

Changes:

• Introduces allowNull?: boolean schema option and SchemaType#allowNull() API, plus a dedicated validator/error message.
• Updates TypeScript inference (InferSchemaType / InferRawDocType) to omit | null when allowNull: false is specified.
• Extends JSON Schema generation and adds runtime + type-level tests covering validation, cloning, updates, discriminators, and JSON schema output.

Reviewed changes

Copilot reviewed 10 out of 13 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
types/schematypes.d.ts	Adds allowNull?: boolean option and allowNull() method typing.
types/inferschematype.d.ts	Adds PathAllowsNull helper and uses it to conditionally include null in inferred optional paths.
types/inferrawdoctype.d.ts	Applies PathAllowsNull to raw doc inference for optional paths.
lib/schemaType.js	Implements SchemaType#allowNull(), required/allowNull interaction checks, JSON schema nullability behavior, and cloning support.
lib/options/schemaTypeOptions.js	Documents and exposes allowNull as a schema type option.
lib/error/messages.js	Adds the default error message for the allowNull validator.
test/types/schema.test.ts	Adds type-level tests for allowNull: false inference.
test/types/schema.create.test.ts	Adds type-level tests for Schema.create() + allowNull: false.
test/schema.validation.test.js	Adds validation behavior tests for allowNull, including interaction with required.
test/schema.test.js	Adds clone and JSON schema tests for allowNull.
test/model.updateOne.test.js	Ensures allowNull validator runs with updateOne() + runValidators.
test/model.findOneAndUpdate.test.js	Ensures allowNull validator runs with findOneAndUpdate() + runValidators.
test/model.discriminator.test.js	Tests discriminator overrides involving allowNull and required.

[AbdelrahmanHafez]

LGTM 👍