Fix the initial comparison to ensure Map gets set

Author: krassowski

jupyter_ydoc/ynotebook.py

@@ -304,7 +304,7 @@ def set(self, value: Dict) -> None:
                 self._ymeta["nbformat_minor"] = nbformat_minor
 
             old_y_metadata = self._ymeta.get("metadata")
-            old_metadata = old_y_metadata.to_py() if old_y_metadata else {}
+            old_metadata = old_y_metadata.to_py() if old_y_metadata else None
             metadata = nb.get("metadata", {})
 
             if metadata != old_metadata: