### 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
"""Metaclass of BaseOperator."""
    @classmethod
    def _apply_defaults(cls, func: T) -> T:
        """
        Function decorator that Looks for an argument named "default_args", and
        fills the unspecified arguments from it.
        Since python2.* isn't clear about which arguments are missing when
        calling a function, and that this can be quite confusing with multi-level
        inheritance and argument defaults, this decorator also alerts with
        specific information about the missing arguments.
        """
        
        
        
        
        sig_cache = signature(func)
        non_optional_args = {
            name
            for (name, param) in sig_cache.parameters.items()
            if param.default == param.empty
            and param.name != 'self'
            and param.kind not in (param.VAR_POSITIONAL, param.VAR_KEYWORD)
        }
        class autostacklevel_warn: