fix:AttributeError in _openai_utils.py with Azure OpenAI Async Streaming #4000 (#4001)

# Description
#4000


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---------

Co-authored-by: Hanchi Wang <[email protected]>

Author: kamilamin123

src/promptflow-tracing/promptflow/tracing/_openai_utils.py

@@ -253,7 +253,7 @@ def get_generated_message(self):
         chunks = []
         role = "assistant"
         for item in self._response:
-            if item.choices and item.choices[0].delta.content:
+            if item.choices and item.choices[0].delta and getattr(item.choices[0].delta, "content", None):
                 chunks.append(item.choices[0].delta.content)
                 role = item.choices[0].delta.role or role
         return {"content": "".join(chunks), "role": role} if chunks else None