fix null serialization for empty hub stats fields

hub-stats.py

@@ -119,11 +119,11 @@ def parse_link_header(link_header):
 
 
 def to_json_string(x):
-    return (
-        json.dumps(x)
-        if isinstance(x, (dict, list))
-        else str(x) if x is not None else None
-    )
+    if isinstance(x, (dict, list)):
+        return json.dumps(x)
+    if x is None or pd.isna(x):
+        return None
+    return str(x)
 
 
 def stringify_nested_columns(df):