fix(output): keep schema-backed party nulls on the wire

[mstykow]

Summary

• keep schema-backed party keys (type, role, name, email, url) present as explicit null when absent so Provenant matches ScanCode's documented Party shape and current package outputs more closely
• extend the JSON writer contract test to cover both top-level packages[] and nested files[].package_data[] party serialization

Issues

• Covers: follow-up schema-shape audit for ScanCode-backed fields
• Closes:

Scope and exclusions

• Included:
  • OutputParty serialization shape for ScanCode-backed fields
  • focused public JSON contract assertions for package and package_data party objects
• Explicit exclusions:
  • no broader package/dependency null-vs-omitted overhaul
  • no changes to runtime-only license-match fields in this PR

Intentional differences from Python

• None.

Follow-up work

• Created or intentionally deferred:
  • continue the broader schema/runtime audit for remaining non-party fields where ScanCode docs are stale or incomplete

Expected-output fixture changes

• Files changed: none
• Why the new expected output is correct:
  • the change is covered by a focused writer contract test rather than golden fixture refreshes, and it aligns the emitted party shape with both the formal ScanCode Party schema and upstream package outputs that use explicit null for absent party fields