[Bug]: generate_init_message fails for multimodal model.

[BeibinLi]

Describe the bug

Note that the second method (in the bottom) does not work today, because of recent updates in generate_init_message

image_agent = MultimodalConversableAgent(
    name="image-explainer",
    max_consecutive_auto_reply=10,
    llm_config={"config_list": config_list_4v, "temperature": 0.5, "max_tokens": 300},
)

user_proxy = autogen.UserProxyAgent(
    name="User_proxy",
    system_message="A human admin.",
    human_input_mode="NEVER",  # Try between ALWAYS or NEVER
    max_consecutive_auto_reply=0,
    code_execution_config={
        "use_docker": False
    },  # Please set use_docker=True if docker is available to run the generated code. Using docker is safer than running the generated code directly.
)

# Ask the question with an image
user_proxy.initiate_chat(
    image_agent,
    message="""What's the breed of this dog?
<img https://th.bing.com/th/id/R.422068ce8af4e15b0634fe2540adea7a?rik=y4OcXBE%2fqutDOw&pid=ImgRaw&r=0>.""",
)

# Equivalent
image_agent.clear_history()
user_proxy.initiate_chat(
    image_agent,
    message=[
        {
        "type": "text",
        "text": "What's the breed of this dog?"
        },
        {
        "type": "image_url",
        "image_url": {
            "url": get_pil_image("https://th.bing.com/th/id/R.422068ce8af4e15b0634fe2540adea7a?rik=y4OcXBE%2fqutDOw&pid=ImgRaw&r=0")
                },
        },
        {
        "type": "text",
        "text": "."
        },
    ]
)

Error message:

File ~/autogen/autogen/agentchat/conversable_agent.py:622, in ConversableAgent.send(self, message, recipient, request_reply, silent)
    620     recipient.receive(message, self, request_reply, silent)
    621 else:
--> 622     raise ValueError(
    623         "Message can't be converted into a valid ChatCompletion message. Either content or function_call must be provided."
    624     )

ValueError: Message can't be converted into a valid ChatCompletion message. Either content or function_call must be provided.

Steps to reproduce

No response

Model Used

No response

Expected Behavior

No response

Screenshots and logs

No response

Additional Information

No response