Add last_edited_time and created_time props to NotionDBLoader (#11020)

# Description

Adds logic for NotionDBLoader to correctly populate `last_edited_time`
and `created_time` fields from [page
properties](https://developers.notion.com/reference/page#property-value-object).

There are no relevant tests for this code to be updated.

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/langchain/langchain/document_loaders/notiondb.py

@@ -121,11 +121,15 @@ class NotionDBLoader(BaseLoader):
                     else []
                 )
             elif prop_type == "date":
-                value = prop_data["date"]
-            elif prop_type == "created_time":
-                value = prop_data["created_time"]
+                value = prop_data["date"] if prop_data["date"] else None
             elif prop_type == "last_edited_time":
-                value = prop_data["last_edited_time"]
+                value = (
+                    prop_data["last_edited_time"]
+                    if prop_data["last_edited_time"]
+                    else None
+                )
+            elif prop_type == "created_time":
+                value = prop_data["created_time"] if prop_data["created_time"] else None
             else:
                 value = None