### 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)

tests/serialization/test_dag_serialization.py
"""OperatorLink not registered via Plugins nor a built-in OperatorLink"""
            name = 'My Link'
            def get_link(self, operator, dttm):
                return 'https://www.google.com'
        class MyOperator(BaseOperator):
            """Just a DummyOperator using above defined Extra Operator Link"""
            operator_extra_links = [TaskStateLink()]
            def execute(self, context):
                pass
        with DAG(dag_id='simple_dag', start_date=datetime(2019, 8, 1)) as dag:
            MyOperator(task_id='blah')
        serialized_dag = SerializedDAG.to_dict(dag)
        with self.assertLogs("airflow.serialization.serialized_objects", level="ERROR") as log_output:
            SerializedDAG.from_dict(serialized_dag)
            received_logs = log_output.output[0]
            expected_err_msg = (
                "Operator Link class 'tests.serialization.test_dag_serialization.TaskStateLink' "
                "not registered"
            )
            assert expected_err_msg in received_logs
    def test_extra_serialized_field_and_multiple_operator_links(self):