### Body

Add unit test for changes in https://github.com/apache/airflow/pull/17305

### Committer

- [X] I acknowledge that I am a maintainer/committer of the Apache Airflow project.
tests/jobs/test_backfill_job.py
with dag_maker(
            'test_backfill_fill_blanks',
        ) as dag:
            op1 = DummyOperator(task_id='op1')
            op2 = DummyOperator(task_id='op2')
            op3 = DummyOperator(task_id='op3')
            op4 = DummyOperator(task_id='op4')
            op5 = DummyOperator(task_id='op5')
            op6 = DummyOperator(task_id='op6')
        dr = dag_maker.create_dagrun()
        executor = MockExecutor()
        session = settings.Session()
        tis = dr.get_task_instances()
        for ti in tis:
            if ti.task_id == op1.task_id: