Fix handling of default empty string ("") in CallbackData filter (#1493)

* 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 = ""

* Create 1493.bugfix.rst

* Update callback_data.py

Fixed an issue that prevented unpacking None values.

* Added tests for CallbackData

* Update tests/test_filters/test_callback_data.py

Co-authored-by: Oleg A. <t0rr@mail.ru>

* Update test_callback_data.py

* Update callback_data.py

* Update 1493.bugfix.rst

---------

Co-authored-by: Oleg A. <t0rr@mail.ru>

tests/test_filters/test_callback_data.py

@@ -160,6 +160,13 @@ class TestCallbackData:
         assert MyCallback3.unpack("test3:experiment:42") == MyCallback3(bar=42)
         assert MyCallback3.unpack("test3:spam:42") == MyCallback3(foo="spam", bar=42)
 
+        class MyCallback4(CallbackData, prefix="test4"):
+            foo: Optional[str] = ""
+            bar: Optional[str] = None
+
+        assert MyCallback4.unpack("test4::") == MyCallback4(foo="", bar=None)
+        assert MyCallback4.unpack("test4::") == MyCallback4()
+
     @pytest.mark.parametrize(
         "hint",
         [