Add support for nullable fields in callback data

This update extends the callback data handling by adding support for nullable fields. The code now uses the Python typing structures `Optional` and `Union` to parse such fields correctly. A helper function `_check_field_is_nullable` has been added to assist in efficiently checking if a given field is nullable.

tests/test_fsm/test_state.py

@@ -1,6 +1,7 @@
-import pytest
 import sys
 
+import pytest
+
 from aiogram.fsm.state import State, StatesGroup, any_state
 
 PY312_OR_GREATER = sys.version_info >= (3, 12)