### 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
"""Serialization and deserialization should work for every DAG and Operator."""
        dags = collect_dags()
        serialized_dags = {}
        for _, v in dags.items():
            dag = SerializedDAG.to_dict(v)
            SerializedDAG.validate_schema(dag)
            serialized_dags[v.dag_id] = dag
        
        self.validate_serialized_dag(serialized_dags['simple_dag'], serialized_simple_dag_ground_truth)
    @parameterized.expand(
        [
            (
                cron_timetable("0 0 * * *"),
                {
                    "type": "airflow.timetables.interval.CronDataIntervalTimetable",
                    "value": {"expression": "0 0 * * *", "timezone": "UTC"},
                },
            ),
            (
                CustomSerializationTimetable("foo"),
                {
                    "type": "tests.test_utils.timetables.CustomSerializationTimetable",
                    "value": {"value": "foo"},
                },
            ),
        ],
    )
    def test_dag_serialization_to_timetable(self, timetable, serialized_timetable):