**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
if task_instances is None:
                pass
            elif update_extras:
                self.ti_extras.update(task_instances[i])
            else:
                self.ti_init.update(task_instances[i])
            ti = TaskInstance(task=tasks[i], **self.ti_init)
            for key, value in self.ti_extras.items():
                setattr(ti, key, value)
            session.add(ti)
            if single_dag_run is False:
                dr = DagRun(
                    dag_id=dag_id,
                    run_id=f"TEST_DAG_RUN_ID_{i}",
                    execution_date=self.ti_init["execution_date"],
                    run_type=DagRunType.MANUAL.value,
                    state=dag_run_state,
                )
                session.add(dr)
        if single_dag_run:
            dr = DagRun(
                dag_id=dag_id,
                run_id="TEST_DAG_RUN_ID",
                execution_date=self.default_time,
                run_type=DagRunType.MANUAL.value,
                state=dag_run_state,
            )
            session.add(dr)
        session.commit()
class TestGetTaskInstance(TestTaskInstanceEndpoint):