fix(lib): treat null message content as empty in parse_response

[chenlichao]

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Fixes #3840

parse_response() in src/openai/lib/_parsing/_responses.py iterates output.content without a null guard, so a message output item with content: null raises TypeError: 'NoneType' object is not iterable instead of parsing:

client.responses.parse(model="...", input="...")
# API returns output: [{"id": ..., "type": "message", "role": "assistant",
#   "status": "completed", "content": null}]
# -> TypeError: 'NoneType' object is not iterable

Null reaches the parser because stream events are built without validation, and the same payload also passes the non-streaming path (reproduced with a mocked transport, both sync and async). This is the same class of bug as null output, fixed in #3345.

The fix applies the same or [] idiom used at line 61 for null output (one line). The change is in src/openai/lib/, which per CONTRIBUTING.md is handwritten and never modified by the generator, so no custom-code budget impact.

Additional context & links

• Follow-up to parse_response crashes with TypeError when response.output is null in response.completed event (chatgpt.com Codex backend) #3325 / fix: preserve finalized output on null response completion #3345 (null output), one level deeper: message content.
• Regression test added in tests/lib/responses/test_null_output.py next to the existing null-output tests, mirroring the test_parse_response_with_null_output pattern from fix: preserve finalized output on null response completion #3345.
• Verification: reproduced the TypeError on current main; new test fails without the fix and passes with it; full tests/lib/responses/ suite passes (96 passed); ruff check clean on both files; git diff --check clean. Diff is 2 files, +31/−2.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-11T17:06:43.149789Z	617a3d8	PR opened
🔒 Security Review	✅ Completed 2026-09-11T17:08:10.377074Z	617a3d8	PR opened

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