fix: coerce undefined values like null when serializing

[cpruijsen]

Fixes #680

Summary

Serialize undefined the same way as null: coerce it to the falsy value of the declared type instead of throwing. The reported case is ['test1', 'test2', 'test3', undefined] with items: { type: 'string' }, which threw TypeError: Cannot read properties of undefined (reading 'toString'). Objects, integer, number and the date formats threw in the same positions.

Each === null coercion guard in the generated code in index.js is now == null. In lib/serializer.js, asInteger and asNumber get a nullish early return, and the date serializers use == null. This follows the review of #681: "all functions need to conform to this behavior (nullish values get coerced to the falsy value of the corresponding type)".

Unchanged:

• Object properties set to undefined are still skipped, as in JSON.stringify({ a: undefined }). A required property set to undefined still throws.
• anyOf / oneOf still throw the schema mismatch for undefined, because no branch can be chosen.
• format: 'unsafe'.

One output change for input that did not throw before: a nullable: true object schema with no properties now gives null for undefined instead of {}, the same as for null.

The alternative is to emit null for undefined array items, as JSON.stringify([undefined]) does. I did not take it because this library already coerces null to the type's falsy value, and #681 settled on one rule for both. The README "Nullable" section only describes null. I can add undefined there if you want it documented.

The new tests in test/missing-values.test.js fail without the source change and pass with it, except the cases that pin existing behaviour.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct