### 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/models/baseoperator.py
self,
        upstream: bool = False,
        found_descendants: Optional[Set[str]] = None,
    ) -> Set[str]:
        """Get a flat set of relatives' ids, either upstream or downstream."""
        if not self._dag:
            return set()
        if not found_descendants:
            found_descendants = set()
        relative_ids = self.get_direct_relative_ids(upstream)
        for relative_id in relative_ids:
            if relative_id not in found_descendants:
                found_descendants.add(relative_id)
                relative_task = self._dag.task_dict[relative_id]
                relative_task.get_flat_relative_ids(upstream, found_descendants)
        return found_descendants
    def get_flat_relatives(self, upstream: bool = False):
        """Get a flat list of relatives, either upstream or downstream."""
        if not self._dag:
            return set()
        from airflow.models.dag import DAG
        dag: DAG = self._dag
        return list(map(lambda task_id: dag.task_dict[task_id], self.get_flat_relative_ids(upstream)))
    def run(