### 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
executor_config={"pod_override": executor_config_pod},
            doc_md="### Task Tutorial Documentation",
        )
        return {'simple_dag': dag}
def make_user_defined_macro_filter_dag():
    """Make DAGs with user defined macros and filters using locally defined methods.
    For Webserver, we do not include ``user_defined_macros`` & ``user_defined_filters``.
    The examples here test:
        (1) functions can be successfully displayed on UI;
        (2) templates with function macros have been rendered before serialization.
    """
    def compute_next_execution_date(dag, execution_date):
        return dag.following_schedule(execution_date)
    default_args = {'start_date': datetime(2019, 7, 10)}
    dag = DAG(
        'user_defined_macro_filter_dag',
        default_args=default_args,
        user_defined_macros={
            'next_execution_date': compute_next_execution_date,
        },
        user_defined_filters={'hello': lambda name: f'Hello {name}'},
        catchup=False,
    )
    BashOperator(
        task_id='echo',
        bash_command='echo "{{ next_execution_date(dag, execution_date) }}"',
        dag=dag,
    )
    return {dag.dag_id: dag}
def collect_dags(dag_folder=None):