### 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
you add support for DAG serialization - you should add the field to
 `airflow/serialization/schema.json` - they should have correct type defined there.
 Note that we do not support versioning yet so you should only add optional fields to BaseOperator.
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
                         """
    def test_task_group_serialization(self):
        """
        Test TaskGroup serialization/deserialization.
        """
        from airflow.operators.dummy import DummyOperator
        from airflow.utils.task_group import TaskGroup
        execution_date = datetime(2020, 1, 1)
        with DAG("test_task_group_serialization", start_date=execution_date) as dag:
            task1 = DummyOperator(task_id="task1")
            with TaskGroup("group234") as group234:
                _ = DummyOperator(task_id="task2")
                with TaskGroup("group34") as group34:
                    _ = DummyOperator(task_id="task3")
                    _ = DummyOperator(task_id="task4")
            task5 = DummyOperator(task_id="task5")
            task1 >> group234
            group34 >> task5
        dag_dict = SerializedDAG.to_dict(dag)
        SerializedDAG.validate_schema(dag_dict)
        json_dag = SerializedDAG.from_json(SerializedDAG.to_json(dag))
        self.validate_deserialized_dag(json_dag, dag)
        serialized_dag = SerializedDAG.deserialize_dag(SerializedDAG.serialize_dag(dag))
        assert serialized_dag.task_group.children
        assert serialized_dag.task_group.children.keys() == dag.task_group.children.keys()
        def check_task_group(node):