**Apache Airflow version**: 2.0.1

**Environment**: Docker on Linux Mint 20.1, image based on apache/airflow:2.0.1-python3.8

**What happened**:

I'm using the airflow API and the following exception occurred:

```python
>>> import json
>>> import requests
>>> from requests.auth import HTTPBasicAuth
>>> payload = {"dag_ids": ["{my_dag_id}"]}
>>> r = requests.post("https://localhost:8080/api/v1/dags/~/dagRuns/~/taskInstances/list", auth=HTTPBasicAuth('username', 'password'), data=json.dumps(payload), headers={'Content-Type': 'application/json'})
>>> r.status_code
500
>>> print(r.text)
{
  "detail": "None is not of type 'string'\n\nFailed validating 'type' in schema['allOf'][0]['properties'][
'task_instances']['items']['properties']['operator']:\n    {'type': 'string'}\n\nOn instance['task_instanc
es'][5]['operator']:\n    None",
  "status": 500,
  "title": "Response body does not conform to specification",
  "type": "https://airflow.apache.org/docs/2.0.1/stable-rest-api-ref.html#section/Errors/Unknown"
}
None is not of type 'string'

Failed validating 'type' in schema['allOf'][0]['properties']['task_instances']['items']['properties']['ope
rator']:
    {'type': 'string'}

On instance['task_instances'][5]['operator']:
    None
```
This happens on all the "old" task instances before upgrading to 2.0.0
There is no issue with new task instances created after the upgrade.

<!-- (please include exact error messages if you can) -->

**What do you think went wrong?**:
The `operator` column was introduced in 2.0.0. But during migration, all the existing database entries are filled with `NULL` values. So I had to execute this manually in my database
```sql
UPDATE task_instance SET operator = 'NoOperator' WHERE operator IS NULL;
```

**How to reproduce it**:

* Run airflow 1.10.14
* Create a DAG with multiple tasks and run them
* Upgrade airflow to 2.0.0 or 2.0.1
* Make the API call as above

**Anything else we need to know**:
Similar to https://github.com/apache/airflow/issues/13799 but not exactly the same

tests/api_connexion/endpoints/test_task_instance_endpoint.py
},
                ],
                "example_python_operator",
                {
                    "only_failed": True,
                },
                2,
            ),
        ]
    )
    @provide_session
    def test_should_respond_200(
        self, _, main_dag, task_instances, request_dag, payload, expected_ti, session
    ):
        self.create_task_instances(
            session,
            dag_id=main_dag,
            task_instances=task_instances,
            update_extras=False,
            single_dag_run=False,
        )
        self.app.dag_bag.sync_to_db()  
        response = self.client.post(
            f"/api/v1/dags/{request_dag}/clearTaskInstances",
            environ_overrides={"REMOTE_USER": "test"},
            json=payload,
        )
        assert response.status_code == 200
        assert len(response.json["task_instances"]) == expected_ti
    def test_should_respond_200_with_reset_dag_run(self, session):