### 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
"""
        Tests serialize_operator, make sure the deps is in order
        """
        from airflow.operators.dummy import DummyOperator
        from airflow.sensors.external_task import ExternalTaskSensor
        execution_date = datetime(2020, 1, 1)
        with DAG(dag_id="test_deps_sorted", start_date=execution_date) as dag:
            task1 = ExternalTaskSensor(
                task_id="task1",
                external_dag_id="external_dag_id",
                mode="reschedule",
            )
            task2 = DummyOperator(task_id="task2")
            task1 >> task2
        serialize_op = SerializedBaseOperator.serialize_operator(dag.task_dict["task1"])
        deps = serialize_op["deps"]
        assert deps == [
            'airflow.ti_deps.deps.not_in_retry_period_dep.NotInRetryPeriodDep',
            'airflow.ti_deps.deps.not_previously_skipped_dep.NotPreviouslySkippedDep',
            'airflow.ti_deps.deps.prev_dagrun_dep.PrevDagrunDep',
            'airflow.ti_deps.deps.ready_to_reschedule.ReadyToRescheduleDep',
            'airflow.ti_deps.deps.trigger_rule_dep.TriggerRuleDep',
        ]
    def test_task_group_sorted(self):