Backfill: Don't create a DagRun if no tasks match task regex (#16461)

Backfill should not create a DagRun in case there is no any task that matches the regex.

closes: #16460
(cherry picked from commit f2c79b2)

airflow/cli/commands/dag_command.py

@@ -78,6 +78,10 @@ def dag_backfill(args, dag=None):
         dag = dag.partial_subset(
             task_ids_or_regex=args.task_regex, include_upstream=not args.ignore_dependencies
         )
+        if not dag.task_dict:
+            raise AirflowException(
+                f"There are no tasks that match '{args.task_regex}' regex. Nothing to run, exiting..."
+            )
 
     run_conf = None
     if args.conf: