Fix handling of default empty string ("") in CallbackData filter.

Description

Previously, the filter did not activate if a field in the CallbackData class had a default value set to an empty string (""). This update fixes that behavior, ensuring the filter functions correctly in such cases.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

from aiogram import Router, F
from aiogram.filters.callback_data import CallbackData
from aiogram.types import CallbackQuery


class MyCallbackData(CallbackData, prefix="MyCallbackData"):
    input1: str
    input2: str = ""


router = Router()


@router.callback_query(MyCallbackData.filter(F.input1 == "demo"))
async def my_callback(query: CallbackQuery, callback_data: MyCallbackData):

    print("input2 =", callback_data.input2)

Test Configuration:

• Operating System: Windows 10
• Python version: 3.12.2

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes