fix: accept null for nullable tuple items - #908

When items is a list of schemas, each item is checked with buildArrayTypeCondition() before it is serialized. That check ignores nullable, so a tuple item such as { type: 'integer', nullable: true } throws Item at 0 does not match schema definition. for null. A nullable single items schema or object property already accepts null.

This change adds buildTupleItemCondition(), which lets null through when the item schema has nullable: true. buildValue() already serializes it as null. Both the inlined and the function-wrapped tuple paths use the new helper. Non-nullable tuple items still reject null.

Fixes #907

Checklist

• run npm run test && npm run benchmark --if-present (tests pass; benchmark not run)
• 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