**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
#
#
import datetime as dt
from unittest import mock
import pytest
from parameterized import parameterized
from airflow.models import DagBag, DagRun, SlaMiss, TaskInstance
from airflow.security import permissions
from airflow.utils.platform import getuser
from airflow.utils.session import provide_session
from airflow.utils.state import State
from airflow.utils.timezone import datetime
from airflow.utils.types import DagRunType
from tests.test_utils.api_connexion_utils import assert_401, create_user, delete_user
from tests.test_utils.db import clear_db_runs, clear_db_sla_miss
DEFAULT_DATETIME_1 = datetime(2020, 1, 1)
DEFAULT_DATETIME_STR_1 = "2020-01-01T00:00:00+00:00"
DEFAULT_DATETIME_STR_2 = "2020-01-02T00:00:00+00:00"
@pytest.fixture(scope="module")
def configured_app(minimal_app_for_api):