fix: function is not found in the tools_dict

[zakahan]

fix issues #1532

Problem

This issue is caused by the following reasons:
When using certain models (such as doubao-1-5-pro-256k-250115) in the context of stream=True and tool invocation, an empty funciton_call may occur. Specifically, the name field of function_call is None or empty string. This PR resolves this bug by filtering this field in the function.
As shown in the figure:

Solution

Filter out empty function calls in advance in handle_function_calls_async

Testing plan

In the scenario of the current version (google-adk==1.9.0), when executing the following code using adk web (please add the two environment variables ark_api_key and api_base_url), this bug may be reproduced (not guaranteed, but highly likely).
I only tried this model and found that this bug can be reproduced relatively stably. For other issues mentioned in #1532, they should be similar, but I don't have his model.

import os
import random

from google.adk.agents import Agent
from google.adk.models.lite_llm import LiteLlm


def create_litellm_model(
    model: str = "doubao-1-5-pro-256k-250115",
    api_key: str = os.getenv("ARK_API_KEY"),
    api_base: str = os.getenv("API_BASE_URL")
):
    return LiteLlm(
        model=f"openai/{model}",
        api_key=api_key,
        api_base=api_base,
    )

def get_lucky_number():
    """
    Get a lucky number between 1 and 100.
    :return: int
    """
    return random.randint(1, 100)

root_agent = Agent(
    model = create_litellm_model(),
    name = "ChatBot",
    description="This is a chatbot",
    instruction="You are a chatbot and you can answer users' questions.",
    tools=[get_lucky_number],
)

Before fixing:

After fixing: