fix(event-sources): handle dynamodb null type as none, not bool (#929)

Author: Michael Brewer

Diff for: aws_lambda_powertools/utilities/data_classes/dynamo_db_stream_event.py

@@ -106,14 +106,13 @@ def ns_value(self) -> Optional[List[str]]:
         return self.get("NS")
 
     @property
-    def null_value(self) -> Optional[bool]:
+    def null_value(self) -> None:
         """An attribute of type Null.
 
         Example:
             >>> {"NULL": True}
         """
-        item = self.get("NULL")
-        return None if item is None else bool(item)
+        return None
 
     @property
     def s_value(self) -> Optional[str]:

Diff for: tests/functional/test_data_classes.py

@@ -544,6 +544,7 @@ def test_dynamo_attribute_value_null_value():
     attribute_value = AttributeValue(example_attribute_value)
 
     assert attribute_value.get_type == AttributeValueType.Null
+    assert attribute_value.null_value is None
     assert attribute_value.null_value == attribute_value.get_value