Fix some dag_runs removed unintentionally (#13576)

* Fix last removal of dag_run with None as dag_id

* Fix quotes

Author: apilaskowski

composer/workflows/airflow_db_cleanup.py

@@ -360,7 +360,8 @@ def build_query(
 
     logging.info("INITIAL QUERY : " + str(query))
 
-    if dag_id:
+    if hasattr(airflow_db_model, 'dag_id'):
+        logging.info("Filtering by dag_id: " + str(dag_id))
         query = query.filter(airflow_db_model.dag_id == dag_id)
 
     if airflow_db_model == DagRun: