Fix for last_modified being None

jupyter_collaboration/rooms.py

@@ -187,7 +187,9 @@ async def _on_content_change(self, event: str, args: dict[str, Any]) -> None:
                 event (str): Type of change.
                 args (dict): A dictionary with format, type, last_modified.
         """
-        if event == "metadata" and self._last_modified < args["last_modified"]:
+        if event == "metadata" and (
+            self._last_modified is None or self._last_modified < args["last_modified"]
+        ):
             self.log.info("Out-of-band changes. Overwriting the content in room %s", self._room_id)
             self._emit(LogLevel.INFO, "overwrite", "Out-of-band changes. Overwriting the room.")