[Bug]: Gemini Tool Call with no arguments raises "function_call missing" exception

What happened?

I have a chatbot that calls a function with no arguments. Calling the function with OpenAI models works just fine. When using Gemini 1.5 pro, the tool call is made and the results are written to the message history, but upon feeding the messages with the tool call results to the LLM, I get this error:

Exception: Unable to convert openai tool calls={'role': 'assistant', 'content': '', 'tool_calls': [{'id': 'call_2c384bc6-de46- 
4f29-8adc-60dd5805d305', 'function': {'name': 'Get-FAQ', 'arguments': '{}'}, 'type': 'function'}]} to gemini tool calls. 
Received error=function_call missing. Received tool call with 'type': 'function'. No function call in argument - {'id': 
'call_2c384bc6-de46-4f29-8adc-60dd5805d305', 'function': {'name': 'Get-FAQ', 'arguments': '{}'}, 'type': 'function'}

Looks like the problem is in litellm/llms/prompt_templates:

def _gemini_tool_call_invoke_helper(
    function_call_params: ChatCompletionToolCallFunctionChunk,
) -> Optional[litellm.types.llms.vertex_ai.FunctionCall]:
    name = function_call_params.get("name", "") or ""
    arguments = function_call_params.get("arguments", "")
    arguments_dict = json.loads(arguments)
    function_call: Optional[litellm.types.llms.vertex_ai.FunctionCall] = None
    for k, v in arguments_dict.items():
        inferred_protocol_value = infer_protocol_value(value=v)
        _field = litellm.types.llms.vertex_ai.Field(
            key=k, value={inferred_protocol_value: v}
        )
        _fields = litellm.types.llms.vertex_ai.FunctionCallArgs(fields=_field)
        function_call = litellm.types.llms.vertex_ai.FunctionCall(
            name=name,
            args=_fields,
        )
    return function_call

At line 946, function_call variable is initiated with None. In the next line, since there are no args, arguments_dict.items() is an empty list and the function ultimately returns None as if there is no function call at all.

Relevant log output

Traceback (most recent call last):
  File "venv3/lib/python3.11/site-packages/litellm/main.py", line 481, in acompletion
    response = await init_response
               ^^^^^^^^^^^^^^^^^^^
  File "venv3/lib/python3.11/site-packages/litellm/llms/vertex_ai_and_google_ai_studio/gemini/vertex_and_google_ai_studio_gemini.py", line 1229, in async_completion
    request_body = await async_transform_request_body(**data)  # type: ignore
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv3/lib/python3.11/site-packages/litellm/llms/vertex_ai_and_google_ai_studio/gemini/transformation.py", line 407, in async_transform_request_body
    return _transform_request_body(
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv3/lib/python3.11/site-packages/litellm/llms/vertex_ai_and_google_ai_studio/gemini/transformation.py", line 326, in _transform_request_body
    raise e
  File "venv3/lib/python3.11/site-packages/litellm/llms/vertex_ai_and_google_ai_studio/gemini/transformation.py", line 297, in _transform_request_body
    content = _gemini_convert_messages_with_history(messages=messages)
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "venv3/lib/python3.11/site-packages/litellm/llms/vertex_ai_and_google_ai_studio/gemini/transformation.py", line 253, in _gemini_convert_messages_with_history
    raise e
  File "venv3/lib/python3.11/site-packages/litellm/llms/vertex_ai_and_google_ai_studio/gemini/transformation.py", line 216, in _gemini_convert_messages_with_history
    convert_to_gemini_tool_call_invoke(assistant_msg)
  File "venv3/lib/python3.11/site-packages/litellm/llms/prompt_templates/factory.py", line 1072, in convert_to_gemini_tool_call_invoke
    raise Exception(
Exception: Unable to convert openai tool calls={'role': 'assistant', 'content': '', 'tool_calls': [{'id': 'call_2c384bc6-de46-4f29-8adc-60dd5805d305', 'function': {'name': 'Get-Mindspark-FAQ', 'arguments': '{}'}, 'type': 'function'}]} to gemini tool calls. Received error=function_call missing. Received tool call with 'type': 'function'. No function call in argument - {'id': 'call_2c384bc6-de46-4f29-8adc-60dd5805d305', 'function': {'name': 'Get-FAQ', 'arguments': '{}'}, 'type': 'function'}

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BerriAI / litellm