### 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
from airflow.kubernetes.pod_generator import PodGenerator
from airflow.models import DAG, Connection, DagBag, TaskInstance
from airflow.models.baseoperator import BaseOperator, BaseOperatorLink
from airflow.operators.bash import BashOperator
from airflow.security import permissions
from airflow.serialization.json_schema import load_dag_schema_dict
from airflow.serialization.serialized_objects import SerializedBaseOperator, SerializedDAG
from airflow.timetables.simple import NullTimetable, OnceTimetable
from tests.test_utils.mock_operators import CustomOperator, CustomOpLink, GoogleLink
from tests.test_utils.timetables import CustomSerializationTimetable, cron_timetable, delta_timetable
executor_config_pod = k8s.V1Pod(
    metadata=k8s.V1ObjectMeta(name="my-name"),
    spec=k8s.V1PodSpec(
        containers=[
            k8s.V1Container(name="base", volume_mounts=[k8s.V1VolumeMount(name="my-vol", mount_path="/vol/")])
        ]
    ),
)
serialized_simple_dag_ground_truth = {
    "__version": 1,
    "dag": {
        "default_args": {
            "__type": "dict",
            "__var": {
                "depends_on_past": False,
                "retries": 1,
                "retry_delay": {"__type": "timedelta", "__var": 300.0},
                "max_retry_delay": {"__type": "timedelta", "__var": 600.0},
                "sla": {"__type": "timedelta", "__var": 100.0},
            },