### 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
r"""
    Given a number of tasks, builds a dependency chain.
    This function accepts values of BaseOperator (aka tasks), EdgeModifiers (aka Labels), XComArg, or lists
    containing any mix of these types (or a mix in the same list). If you want to chain between two lists
    you must ensure they have the same length.
    Using classic operators/sensors:
    .. code-block:: python
        chain(t1, [t2, t3], [t4, t5], t6)
    is equivalent to::
          / -> t2 -> t4 \
        t1               -> t6
          \ -> t3 -> t5 /
    .. code-block:: python
        t1.set_downstream(t2)
        t1.set_downstream(t3)
        t2.set_downstream(t4)
        t3.set_downstream(t5)
        t4.set_downstream(t6)
        t5.set_downstream(t6)
    Using task-decorated functions aka XComArgs:
    .. code-block:: python
        chain(x1(), [x2(), x3()], [x4(), x5()], x6())
    is equivalent to::
          / -> x2 -> x4 \
        x1               -> x6
          \ -> x3 -> x5 /
    .. code-block:: python
        x1 = x1()