types(schema): avoid treating paths with default: null as required

[vkarpov15]

Fix #15878

Summary

Currently, defining a path as { type: String, default: undefined } will correctly treat the path as not required in TypeScript... but { type: String, default: null } will treat the path as required, which is incorrect.

Examples

[Copilot]

Pull request overview

This PR fixes TypeScript type inference for Mongoose schema paths with default: null. Previously, paths with default: null were incorrectly treated as required in TypeScript, while default: undefined correctly treated them as optional. The fix extends the type checking logic to treat both null and undefined defaults consistently as optional paths.

Key Changes:

• Renamed IsPathDefaultUndefined to IsPathDefaultNullish to reflect expanded scope
• Extended type checking to handle default: null and default: () => null patterns
• Added comprehensive TypeScript test coverage for the new behavior

Reviewed changes

Copilot reviewed 1 out of 2 changed files in this pull request and generated 1 comment.

File	Description
types/inferschematype.d.ts	Renamed and expanded the IsPathDefaultNullish type utility to check for both null and undefined defaults (including function-based defaults), and updated all usage sites
test/types/schema.test.ts	Added TypeScript test case gh15878() validating that paths with default: null and default: () => null are correctly inferred as optional types

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[hasezoey]

Aside from the benchmark, looks good to me

[vkarpov15]

I was able to make some small improvements to reduce number of typescript instantiations, but not enough to get back down to 300k. Which is fine because the ts benchmark is failing on 8.x right now with 338k instantiations, and this PR knocks that down to 334k. Small improvement, and I increased the instantiations limit to make the benchmark pass.