fix(httpv1): rebuild a message whose body is empty (#1050)

[JarryShaw]

• Searched for similar pull requests
• Followed the coding style (make pylint, make mypy, make isort) — not run locally
• make test passes, and a test case covers the change — the touched modules only, not the full suite
• Added a changelog entry under docs/source/changelog/ and regenerated CHANGELOG.md, if the change is user-visible

What is the purpose of your pull request?

• fix — corrects a defect

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Description of your pull request and other information

httpv1.HTTP.from_data(info) raised TypeError: can't concat NoneType to bytes for any message with an empty body. read stores an empty body as None, _make_data passed it through, and make concatenated it.

The fix is in _make_data: it now passes b'' to make when the body is None. info.body stays None because test_http_runtime.py and test_reassembly_end_to_end.py assert that value, and make keeps its bytes signature. HTTPv2 does not have this bug: DATA, HEADERS, SETTINGS, GOAWAY, CONTINUATION and unassigned frames with empty payloads already round-trip.

The new test_httpv1_from_data_round_trips_a_message_with_an_empty_body checks four cases: a request with headers, a headerless request, an empty-bodied response, and a response with a body as a regression guard. Without the fix, the first three fail with the TypeError. The changelog entry pin goes from 166 to the measured 167.

Closes #1050

[JarryShaw]

Cross-review verdict on ff6e9a8d0: GOOD TO GO (ran on Sonnet; author Opus).

• Right layer. Only _make_data changes (httpv1.py:358). read and the public info.body is None contract stay as they were, pinned at test_http_runtime.py:41/77/88, test_reassembly_end_to_end.py:167/187 and test_http_unit.py:1062. make behaves as before for None, str and int bodies.
• Byte-exact round trip for 9 messages: a request with an empty body plus headers, a headerless request, a 204, responses and requests with bodies, Content-Length: 0 both ways, a bare 200 OK, and HTTP/1.0.
• HTTPv2 is unaffected. It has no None body path, and empty DATA and SETTINGS frames round-trip.
• New test: errors 3 times on main (TypeError: can't concat NoneType to bytes) and passes on head.
• Changelog: changelog_md.py --check is in step. The pin reads 167, matching a measured 167; main is at 166.

Not breaking. Not checked: chunked and gzip bodies, and the full suite.