### 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
"""Collects DAGs to test."""
    dags = {}
    dags.update(make_simple_dag())
    dags.update(make_user_defined_macro_filter_dag())
    if dag_folder:
        if isinstance(dag_folder, (list, tuple)):
            patterns = dag_folder
        else:
            patterns = [dag_folder]
    else:
        patterns = [
            "airflow/example_dags",
            "airflow/providers/*/example_dags",
            "airflow/providers/*/*/example_dags",
        ]
    for pattern in patterns:
        for directory in glob(f"{ROOT_FOLDER}/{pattern}"):
            dags.update(make_example_dags(directory))
    
    dags = {dag_id: dag for dag_id, dag in dags.items() if not dag.is_subdag}
    return dags
def get_timetable_based_simple_dag(timetable):
    """Create a simple_dag variant that uses timetable instead of schedule_interval."""
    dag = collect_dags(["airflow/example_dags"])["simple_dag"]
    dag.timetable = timetable
    dag.schedule_interval = timetable.summary
    return dag
def serialize_subprocess(queue, dag_folder):