### 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
def validate_deserialized_task(
        self,
        serialized_task,
        task,
    ):
        """Verify non-airflow operators are casted to BaseOperator."""
        assert isinstance(serialized_task, SerializedBaseOperator)
        assert not isinstance(task, SerializedBaseOperator)
        assert isinstance(task, BaseOperator)
        fields_to_check = task.get_serialized_fields() - {
            
            '_task_type',
            'subdag',
            
            '_log',
            
            'template_fields',
            
            'on_failure_callback',
            'on_success_callback',
            'on_retry_callback',
            
            'resources',
        }
        assert serialized_task.task_type == task.task_type
        assert set(serialized_task.template_fields) == set(task.template_fields)
        assert serialized_task.upstream_task_ids == task.upstream_task_ids
        assert serialized_task.downstream_task_ids == task.downstream_task_ids
        for field in fields_to_check:
            assert getattr(serialized_task, field) == getattr(