Fix bug related to Azure OpenAI model streaming response

Author: frednijsvrt

pydantic_ai_slim/pydantic_ai/models/openai.py

@@ -1173,7 +1173,7 @@ async def _get_event_iterator(self) -> AsyncIterator[ModelResponseStreamEvent]:
 
             # Handle the text part of the response
             content = choice.delta.content
-            if content is not None:
+            if (delta := choice.delta) is not None and (content := delta.content) is not None:
                 maybe_event = self._parts_manager.handle_text_delta(
                     vendor_part_id='content',
                     content=content,