### 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
"""
        Test that when on_failure_callback is passed to the DAG, has_on_failure_callback is stored
        in Serialized JSON blob. And when it is de-serialized dag.has_on_failure_callback is set to True.
        When the callback is not set, has_on_failure_callback should not be stored in Serialized blob
        and so default to False on de-serialization
        """
        dag = DAG(dag_id='test_dag_on_failure_callback_roundtrip', **passed_failure_callback)
        BaseOperator(task_id='simple_task', dag=dag, start_date=datetime(2019, 8, 1))
        serialized_dag = SerializedDAG.to_dict(dag)
        if expected_value:
            assert "has_on_failure_callback" in serialized_dag["dag"]
        else:
            assert "has_on_failure_callback" not in serialized_dag["dag"]
        deserialized_dag = SerializedDAG.from_dict(serialized_dag)
        assert deserialized_dag.has_on_failure_callback is expected_value
    @parameterized.expand(
        [
            (
                ['task_1', 'task_5', 'task_2', 'task_4'],
                ['task_1', 'task_5', 'task_2', 'task_4'],
            ),
            (
                {'task_1', 'task_5', 'task_2', 'task_4'},
                ['task_1', 'task_2', 'task_4', 'task_5'],
            ),
            (
                ('task_1', 'task_5', 'task_2', 'task_4'),
                ['task_1', 'task_5', 'task_2', 'task_4'],
            ),