### 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)

airflow/serialization/serialized_objects.py
"""A JSON serializable representation of TaskGroup."""
    @classmethod
    def serialize_task_group(cls, task_group: TaskGroup) -> Optional[Union[Dict[str, Any]]]:
        """Serializes TaskGroup into a JSON object."""
        if not task_group:
            return None
        
        
        
        serialize_group = {
            "_group_id": task_group._group_id,
            "prefix_group_id": task_group.prefix_group_id,
            "tooltip": task_group.tooltip,
            "ui_color": task_group.ui_color,
            "ui_fgcolor": task_group.ui_fgcolor,
            "children": {
                label: (DAT.OP, child.task_id)
                if isinstance(child, BaseOperator)
                else (DAT.TASK_GROUP, SerializedTaskGroup.serialize_task_group(child))
                for label, child in task_group.children.items()
            },
            "upstream_group_ids": cls._serialize(sorted(task_group.upstream_group_ids)),
            "downstream_group_ids": cls._serialize(sorted(task_group.downstream_group_ids)),
            "upstream_task_ids": cls._serialize(sorted(task_group.upstream_task_ids)),
            "downstream_task_ids": cls._serialize(sorted(task_group.downstream_task_ids)),
        }
        return serialize_group
    @classmethod
    def deserialize_task_group(