Fix bug related to Azure OpenAI streaming response

Author: frednijsvrt

pydantic_ai_slim/pydantic_ai/models/openai.py

@@ -1171,6 +1171,9 @@ async def _get_event_iterator(self) -> AsyncIterator[ModelResponseStreamEvent]:
             except IndexError:
                 continue
 
+            if choice.delta is None:
+                continue
+
             # Handle the text part of the response
             content = choice.delta.content
             if content is not None: