### Apache Airflow version

2.1.3 (latest released)

### Operating System

Debian Buster

### Versions of Apache Airflow Providers

_No response_

### Deployment

Managed Services (Astronomer, Composer, MWAA etc.)

### Deployment details

Running `quay.io/astronomer/ap-airflow:2.1.3-buster` Docker image

### What happened

In the task attributes view of a BashOperator, the `template_ext` row doesn't show any value, while I would expect to see `('.sh', '.bash')`.

![image](https://user-images.githubusercontent.com/6249654/131467817-8620f68d-0f3d-491d-ad38-cbf5b91cdcb3.png)


### What you expected to happen

Expect to see correct `template_ext` value

### How to reproduce

Run a BashOperator and browse to Instance Details --> scroll down to `template_ext` row

### Anything else

Spent a little time trying to figure out what could be the reason, but haven't found the problem yet. Therefore created issue to track.

### Are you willing to submit PR?

- [X] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

tests/serialization/test_dag_serialization.py
if not task_end_date or dag_end_date <= task_end_date:
            
            
            assert "end_date" not in serialized_dag["dag"]["tasks"][0]
        else:
            assert "end_date" in serialized_dag["dag"]["tasks"][0]
        dag = SerializedDAG.from_dict(serialized_dag)
        simple_task = dag.task_dict["simple_task"]
        assert simple_task.end_date == expected_task_end_date
    @parameterized.expand(
        [
            ({"type": "airflow.timetables.simple.NullTimetable", "value": {}}, NullTimetable()),
            (
                {
                    "type": "airflow.timetables.interval.CronDataIntervalTimetable",
                    "value": {"expression": "@weekly", "timezone": "UTC"},
                },
                cron_timetable("0 0 * * 0"),
            ),
            ({"type": "airflow.timetables.simple.OnceTimetable", "value": {}}, OnceTimetable()),
            (
                {
                    "type": "airflow.timetables.interval.DeltaDataIntervalTimetable",
                    "value": {"delta": 86400.0},
                },
                delta_timetable(timedelta(days=1)),
            ),
        ]
    )
    def test_deserialization_timetable(