Update callback_data.py

Allows using a default value in the class which is equal to an empty string ("").

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

aiogram/filters/callback_data.py

@@ -131,7 +131,7 @@ class CallbackData(BaseModel):
         for k, v in zip(names, parts):  # type: str, Optional[str]
             if field := cls.model_fields.get(k):
                 if v == "" and _check_field_is_nullable(field):
-                    v = None
+                    v = field.default
             payload[k] = v
         return cls(**payload)