[EnesKartal]

Fixes #1520.

Summary

• In union_property.py.jinja (transform macro), initialize ns.contains_modified_properties as not property.required and not skip_unset so that when skip_unset=True skips the if isinstance(..., Unset): branch (as in endpoint_macros.py.jinja json_body), no orphan else: is emitted when all union members lack a transform macro.
• When not property.required and skip_unset is true, guard fallback and trailing branches with if not isinstance(..., Unset): / elif not isinstance(..., Unset): / elif isinstance(..., ...): instead of falling into an unconditional assignment or else:.
• Add regression tests in tests/test_templates/test_endpoint_macros.py.
• Verified locally with pytest tests, end_to_end_tests/test_end_to_end.py (all 5 snapshots passing), ruff check, ruff format --check, and mypy openapi_python_client.