set default dag_run to None

csst_dag/dag/_base_dag.py

@@ -113,7 +113,7 @@ class BaseDAG:
                 )
                 this_task["dag_run"] = dag_run
             else:
-                this_task["dag_run"] = dict()
+                this_task["dag_run"] = None
         return task_list
 
     @staticmethod