fix: preserve omitted and null JSON request bodies

[CodingCossack]

Fixes #1425

Problem

An optional nullable primitive request body, for example {type: number, nullable: true} without required: true, generated an endpoint containing an orphan else:, so the generated client did not compile. A syntax-only repair would still be wrong: HTTPX treats json=None as "no body", so an explicit None and an omitted argument would both send an empty body.

Change

Only openapi_python_client/templates/endpoint_macros.py.jinja changes in production code. The JSON body macro now:

1. uses the normal property transform (default skip_unset=False), which avoids the broken union branch;
2. removes the json kwarg when the serialised value is UNSET;
3. replaces the json kwarg with content=b"null" when the value is None.

Presence and nullability stay independent:

Caller input	Request body
Omitted or UNSET	empty
None	null
Value, including 0, False, ""	its JSON

Signature requiredness, nested model serialisation, other body types and the declared Content-Type are unchanged. The 17 golden-record diffs are this boundary logic only.

Tests

New functional tests generate clients from inline OpenAPI 3.0 and 3.1 specs and assert the actual request bytes through httpx.MockTransport, sync and async, for nullable primitives, type lists, anyOf/oneOf, required/optional combinations, date-time, model, array and nested nullable fields, a custom JSON media type, and a query parameter named json_body with strict mypy on the generated package. The file fails on main (generated package does not compile) and passes with this change.

pdm run check passes locally: 480 tests, five snapshots, Ruff and mypy with HTTPX 0.28.1. All 12 new regression tests also pass with installed HTTPX 0.23.1 and 0.27.2. Structured object/array assertions compare decoded JSON, so valid differences in encoder whitespace do not fail the tests; omission, explicit null and primitive values retain exact-byte assertions. An independent review also generated clients from a further set of body shapes (required nullable model, nullable list/date/uuid, Any, single-member unions, form and multipart) and found no regressions.