### 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
task, field
            ), f'{task.dag.dag_id}.{task.task_id}.{field} does not match'
        if serialized_task.resources is None:
            assert task.resources is None or task.resources == []
        else:
            assert serialized_task.resources == task.resources
        
        
        if task.task_type == "SubDagOperator":
            assert serialized_task.subdag is not None
            assert isinstance(serialized_task.subdag, DAG)
        else:
            assert serialized_task.subdag is None
    @parameterized.expand(
        [
            (datetime(2019, 8, 1, tzinfo=timezone.utc), None, datetime(2019, 8, 1, tzinfo=timezone.utc)),
            (
                datetime(2019, 8, 1, tzinfo=timezone.utc),
                datetime(2019, 8, 2, tzinfo=timezone.utc),
                datetime(2019, 8, 2, tzinfo=timezone.utc),
            ),
            (
                datetime(2019, 8, 1, tzinfo=timezone.utc),
                datetime(2019, 7, 30, tzinfo=timezone.utc),
                datetime(2019, 8, 1, tzinfo=timezone.utc),
            ),
            (pendulum.datetime(2019, 8, 1, tz='UTC'), None, pendulum.datetime(2019, 8, 1, tz='UTC')),
        ]
    )
    def test_deserialization_start_date(self, dag_start_date, task_start_date, expected_task_start_date):