### 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
"""Validate pickle in a subprocess."""
    dags = collect_dags(dag_folder)
    for dag in dags.values():
        queue.put(SerializedDAG.to_json(dag))
    queue.put(None)
class TestStringifiedDAGs(unittest.TestCase):
    """Unit tests for stringified DAGs."""
    def setUp(self):
        super().setUp()
        BaseHook.get_connection = mock.Mock(
            return_value=Connection(
                extra=(
                    '{'
                    '"project_id": "mock", '
                    '"location": "mock", '
                    '"instance": "mock", '
                    '"database_type": "postgres", '
                    '"use_proxy": "False", '
                    '"use_ssl": "False"'
                    '}'
                )
            )
        )
        self.maxDiff = None
    def test_serialization(self):