Fix: litellm assistant message empty content convert from empty list to empty string

[javelir]

background: assistant message with tool_calls and empty list content can cause litellm crash for Third-Party Models such as OpenAI, DeepSeek, ollama models, etc.
fix: change empty list content to empty string

messages before fix:
[{'role': 'developer', 'content': 'I can answer your questions about the time and weather in a city.\n\nYou are an agent. Your internal name is "weather_time_agent".\n\n The description about you is "Agent to answer questions about the time and weather in a city."'}, {'role': 'user', 'content': 'what time is it in new york?'}, {'role': 'assistant', 'content': [], 'tool_calls': [dict(function=dict(arguments='{"city": "New York"}', name='get_current_time'), id='f61b9dcb-592a-4a3d-9598-1006bc0cfedf', type='function')]}, {'role': 'tool', 'tool_call_id': 'f61b9dcb-592a-4a3d-9598-1006bc0cfedf', 'content': '{"status": "success", "report": "The current time in New York is 2025-04-12 07:35:46 EDT-0400"}'}]

messages after fix -- messages[2]['content']
[{'role': 'developer', 'content': 'I can answer your questions about the time and weather in a city.\n\nYou are an agent. Your internal name is "weather_time_agent".\n\n The description about you is "Agent to answer questions about the time and weather in a city."'}, {'role': 'user', 'content': 'what time is it in new york?'}, {'role': 'assistant', 'content': '', 'tool_calls': [dict(function=dict(arguments='{"city": "New York"}', name='get_current_time'), id='f61b9dcb-592a-4a3d-9598-1006bc0cfedf', type='function')]}, {'role': 'tool', 'tool_call_id': 'f61b9dcb-592a-4a3d-9598-1006bc0cfedf', 'content': '{"status": "success", "report": "The current time in New York is 2025-04-12 07:35:46 EDT-0400"}'}]

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[binnn6]

related issues.
#171
#20
#143

[hangfei]

related issues. #171 #20 #143

it's fixed per the discussion in this post.