### 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 edge_info serialization/deserialization.
        """
        from airflow.operators.dummy import DummyOperator
        from airflow.utils.edgemodifier import Label
        with DAG("test_edge_info_serialization", start_date=datetime(2020, 1, 1)) as dag:
            task1 = DummyOperator(task_id="task1")
            task2 = DummyOperator(task_id="task2")
            task1 >> Label("test label") >> task2
        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.edge_info == dag.edge_info
    @parameterized.expand(
        [
            ("poke", False),
            ("reschedule", True),
        ]
    )
    def test_serialize_sensor(self, mode, expect_custom_deps):
        from airflow.sensors.base import BaseSensorOperator
        class DummySensor(BaseSensorOperator):