### 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.owner = owner
        self.email = email
        self.email_on_retry = email_on_retry
        self.email_on_failure = email_on_failure
        self.start_date = start_date
        if start_date and not isinstance(start_date, datetime):
            self.log.warning("start_date for %s isn't datetime.datetime", self)
        elif start_date:
            self.start_date = timezone.convert_to_utc(start_date)
        self.end_date = end_date
        if end_date:
            self.end_date = timezone.convert_to_utc(end_date)
        if trigger_rule == "dummy":
            warnings.warn(
                "dummy Trigger Rule is deprecated. Please use `TriggerRule.ALWAYS`.",
                DeprecationWarning,
                stacklevel=2,
            )
            trigger_rule = TriggerRule.ALWAYS
        if trigger_rule == "none_failed_or_skipped":
            warnings.warn(
                "none_failed_or_skipped Trigger Rule is deprecated. "
                "Please use `none_failed_min_one_success`.",
                DeprecationWarning,
                stacklevel=2,
            )
            trigger_rule = TriggerRule.NONE_FAILED_MIN_ONE_SUCCESS
        if not TriggerRule.is_valid(trigger_rule):
            raise AirflowException(
                "The trigger_rule must be one of {all_triggers},"