fix jsonata toMillis

Author: Louis Auneau

src/jsonata/functions.py

@@ -2107,7 +2107,7 @@ def datetime_to_millis(timestamp: Optional[str], picture: Optional[str]) -> Opti
                 dt = datetime.datetime.fromisoformat(timestamp)
 
             if dt.tzinfo is None:
-                return dt.replace(tzinfo=datetime.timezone.utc)
+                dt = dt.replace(tzinfo=datetime.timezone.utc)
 
             return int(dt.timestamp() * 1000)
             # try: