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_filters/test_callback_data.py

@@ -1,7 +1,7 @@
 from decimal import Decimal
 from enum import Enum, auto
 from fractions import Fraction
-from typing import Optional
+from typing import Optional, Union
 from uuid import UUID
 
 import pytest
@@ -147,12 +147,19 @@ class TestCallbackData:
         assert MyCallback3.unpack("test3:experiment:42") == MyCallback3(bar=42)
         assert MyCallback3.unpack("test3:spam:42") == MyCallback3(foo="spam", bar=42)
 
-    def test_unpack_optional_wo_default(self):
+    @pytest.mark.parametrize(
+        "hint",
+        [
+            Union[int, None],
+            Optional[int],
+        ],
+    )
+    def test_unpack_optional_wo_default(self, hint):
         """Test CallbackData without default optional."""
 
         class TgData(CallbackData, prefix="tg"):
             chat_id: int
-            thread_id: Optional[int]
+            thread_id: hint
 
         assert TgData.unpack("tg:123:") == TgData(chat_id=123, thread_id=None)