fix(ingestion/airflow-plugin): fixed the failing pipeline

metadata-ingestion-modules/airflow-plugin/src/datahub_airflow_plugin/datahub_listener.py

@@ -414,10 +414,10 @@ def on_task_instance_running(
             f"DataHub listener finished processing notification about task instance start for {task_instance.task_id}"
         )
 
-        if not self.config.materialize_iolets:
+        if not self.config.materialize_iolets and self.graph:
             for outlet in datajob.outlets:
                 if not self.graph.exists(str(outlet)):
-                    logger.warning(f"dataset {str(outlet)} not materialized")
+                    logger.warning(f"Dataset {str(outlet)} not materialized")
 
     def on_task_instance_finish(
         self, task_instance: "TaskInstance", status: InstanceRunResult