fix: fix bug in monitor.attach() for empty network_config (#304)

Author: knakad

src/sagemaker/model_monitor/model_monitoring.py

@@ -1531,6 +1531,7 @@ def attach(cls, monitor_schedule_name, sagemaker_session=None):
         if vpc_config is not None:
             subnets = vpc_config["Subnets"]
 
+        network_config = None
         if network_config_dict:
             network_config = NetworkConfig(
                 enable_network_isolation=network_config_dict["EnableNetworkIsolation"],