TypeScript-Zod coerces nullable date-time null to epoch

[schani]

Summary

The TypeScript-Zod renderer currently emits nullable date-time fields as a union containing z.coerce.date() and z.null(). Because z.coerce.date() accepts null and coerces it to the Unix epoch, a JSON field with null round-trips as "1970-01-01T00:00:00.000Z" instead of null.

Reproduction

This was found while adding the regression fixture in #2879:

{
  "invoices": [
    { "name": "invoice-1", "createdAt": "2023-01-15T10:30:00Z" },
    { "name": "invoice-2", "createdAt": null }
  ]
}

The generated schema includes:

"createdAt": z.union([z.coerce.date(), z.null()])

The second object's createdAt: null is parsed by z.coerce.date() before the null branch can preserve it, so the fixture output differs from the input.

Expected behavior

Nullable date-time fields should preserve null when parsing/serializing.

Notes

#2879 is skipping its new fixture for TypeScript-Zod so the unrelated Dart regression fix can pass CI.