Fix handling of examples with no tools in Gemini

[gabrielibagon]

There is a bug in handling examples with no tools / function definitions. Because the tools array is created like this:

tools = [Tool(function_declarations=func_declarations)]

the following conditional would always submit a tool, since the array has a single element:

        api_response = self.generate_with_backoff(
            client=client,
            contents=inference_data["message"],
            generation_config=GenerationConfig(
                temperature=self.temperature,
            ),
            tools=tools if len(tools) > 0 else None,  # conditional will always resolve to True
        )

This led to the following Gemini API error when a Tool was submitted with no function declarations:

400 Request contains an invalid argument. [detail: "[ORIGINAL ERROR] generic::invalid_argument: The GenerateContentRequest proto is invalid:\n  * tools[0].tool_type: required one_of \'tool_type\' must have one initialized field"

The fix moves the conditional to the creation of the tools array:

        if func_declarations:
            tools = [Tool(function_declarations=func_declarations)]
        else:
            tools = None