fix: fix azure open-4o-08-06 when enable json schema cant process content = "" (#11204)

Co-authored-by: jiaming.liu <jiaming.liu@zkh.com>

api/core/model_runtime/model_providers/azure_openai/llm/llm.py

             # message = cast(AssistantPromptMessage, message)
             message_dict = {"role": "assistant", "content": message.content}
             if message.tool_calls:
+                # fix azure when enable json schema cant process content = "" in assistant fix with None
+                if not message.content:
+                    message_dict["content"] = None
                 message_dict["tool_calls"] = [helper.dump_model(tool_call) for tool_call in message.tool_calls]
         elif isinstance(message, SystemPromptMessage):
             message = cast(SystemPromptMessage, message)