Fix FilterListItem story shows wrong way of resetting a filter

Author: fzaninotto

packages/ra-ui-materialui/src/list/filter/FilterList.stories.tsx

@@ -301,7 +301,7 @@ const BookListAside = () => (
             <FilterList label="Century" icon={<CategoryIcon />}>
                 <FilterListItem
                     label="21st"
-                    value={{ year_gte: 2000, year_lte: null }}
+                    value={{ year_gte: 2000, year_lte: undefined }}
                 />
                 <FilterListItem
                     label="20th"