Fix #7130 - Add None chunk guard in OpenAI create_stream to prevent AttributeError

[Whning0513]

AttributeError in streaming due to missing None check on chunk.model

The BaseOpenAIChatCompletionClient.create_stream() method crashes with AttributeError: 'NoneType' object has no attribute 'model' when the OpenAI stream yields None chunks. This is especially prevalent in compiled binaries (PyInstaller/Nuitka) and causes premature stream termination. The type annotation already indicated chunk could be None, but the code accessed chunk.model before checking for None.

Reported by: abhijaysingh

In the _create_stream_chunks method of _openai_client.py, the async iteration over stream chunks accessed chunk.model (line 919) before any None check. While empty chunk checks existed later in the code, the None guard was missing entirely despite the type annotation at line 893 declaring chunk: ChatCompletionChunk | None = None.

File: autogen/python/packages/autogen-ext/src/autogen_ext/models/openai/_openai_client.py

Added a if chunk is None: continue guard at line 909, right at the top of the async for loop that processes stream chunks. This ensures None chunks (which can be emitted by the OpenAI API as keepalives/heartbeats or under load) are silently skipped before any attribute access occurs.

Low - The change is a single defensive guard that skips None chunks. This aligns with the existing type annotation and does not alter any valid processing behavior. All existing tests continue to pass.

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