Fix OTA URL check syntax (#834)

Author: agners

matter_server/server/device_controller.py

@@ -1024,7 +1024,7 @@ async def _check_node_update(
             node_logger.info("No new update found.")
             return None, None
 
-        if "otaUrl" not in update or update["otaUrl"].strip().length == 0:
+        if "otaUrl" not in update or update["otaUrl"].strip() == "":
             raise UpdateCheckError("Update found, but no OTA URL provided.")
 
         node_logger.info(