Handle None content in OpenAI representation (#2353)

[lntutor]

What does this PR do?

Fixes #2353

Root cause

In bertopic/representation/_openai.py, the generated label was guarded with:

if response and hasattr(response.choices[0].message, "content"):
    label = response.choices[0].message.content.strip().replace("topic: ", "")
else:
    label = "No label returned"

content is always present as a field on the message, so hasattr(...) is always True. When OpenAI/AzureOpenAI blocks a response via the content filter, it returns finish_reason='content_filter' with content=None. The guard passes and None.strip() raises:

AttributeError: 'NoneType' object has no attribute 'strip'

as reported in the issue traceback (at the .content.strip() line).

Fix

Read content first and check it explicitly against None:

content = response.choices[0].message.content if response else None
if content is not None:
    label = content.strip().replace("topic: ", "")
else:
    label = "No label returned"

This reuses the existing "No label returned" placeholder and keeps behavior identical for normal (non-None) responses. There is a single chat-completions code path in this file, so no other call site needs changing.

Test

Adds tests/test_representation/test_openai.py with two lightweight unit tests that call extract_topics directly against a mocked OpenAI client (no model training):

• content=None returns {0: [("No label returned", 1)]} and does not raise (this fails on the previous implementation with the AttributeError from the issue).
• normal content "topic: Pets\n" is stripped and the topic: prefix removed, yielding {0: [("Pets", 1)]}.

The module imports openai at load time, so the tests use pytest.importorskip("openai"), and openai>=1.0.0 is added to the test extra so they run under CI's uv sync --extra test.

Before submitting

• This PR fixes a typo or improves the docs (if yes, ignore all other checks!).
• Did you read the contributor guideline?
• Was this discussed/approved via a Github issue? See AttributeError: 'NoneType' object has no attribute 'strip' when using AzureOpenAI client #2353.
• Did you make sure to update the documentation with your changes (if applicable)? Not applicable.
• Did you write any new necessary tests?