
The list event log endpoint is not working

**Apache Airflow version**: 2.0

**Environment**: Breeze

**What happened**:

Calling the list event logs endpoint `http://localhost:28080/api/v1/eventLogs` returns 500 status code with the below message:

```
{
    "detail": "None is not of type 'string'\n\nFailed validating 'type' in schema['allOf'][0]['properties']['event_logs']['items']['properties']['dag_id']:\n    {'description': 'The DAG ID', 'readOnly': True, 'type': 'string'}\n\nOn instance['event_logs'][0]['dag_id']:\n    None",
    "status": 500,
    "title": "Response body does not conform to specification",
    "type": "https://airflow.readthedocs.io/en/latest/stable-rest-api-ref.html#section/Errors/Unknown"
}
```

**What you expected to happen**:

I expected it to produce a list of event logs:
```
{
    "event_logs": [
        {
            "dag_id": null,
            "event": "cli_webserver",
            "event_log_id": 482,
            "execution_date": null,
            "extra": "{\"host_name\": \"e24b454f002a\", \"full_command\": \"['/usr/local/bin/airflow', 'webserver']\"}",
            "owner": "root",
            "task_id": null,
            "when": "2020-11-11T03:28:48.722814+00:00"
        },
        {
            "dag_id": null,
            "event": "cli_scheduler",
            "event_log_id": 483,
            "execution_date": null,
            "extra": "{\"host_name\": \"e24b454f002a\", \"full_command\": \"['/usr/local/bin/airflow', 'scheduler']\"}",
            "owner": "root",
            "task_id": null,
            "when": "2020-11-11T03:32:18.684231+00:00"
        },
     ],
    "total_entries": 2
}
```

**How to reproduce it**:
1. Start airflow webserver and scheduler in breeze
2. Call the endpoint `http://localhost:28080/api/v1/eventLogs`
3. Check the response

tests/api_connexion/endpoints/test_event_log_endpoint.py
"TEST_EVENT_3",
                    "TEST_EVENT_4",
                    "TEST_EVENT_5",
                    "TEST_EVENT_6",
                    "TEST_EVENT_7",
                    "TEST_EVENT_8",
                    "TEST_EVENT_9",
                    "TEST_EVENT_10",
                ],
            ),
            ("api/v1/eventLogs?limit=1&offset=5", ["TEST_EVENT_6"]),
            ("api/v1/eventLogs?limit=1&offset=1", ["TEST_EVENT_2"]),
            (
                "api/v1/eventLogs?limit=2&offset=2",
                ["TEST_EVENT_3", "TEST_EVENT_4"],
            ),
        ]
    )
    @provide_session
    def test_handle_limit_and_offset(self, url, expected_events, session):
        log_models = self._create_event_logs(10)
        session.add_all(log_models)
        session.commit()
        response = self.client.get(url, environ_overrides={'REMOTE_USER': "test"})
        assert response.status_code == 200
        self.assertEqual(response.json["total_entries"], 10)
        events = [event_log["event"] for event_log in response.json["event_logs"]]
        self.assertEqual(events, expected_events)
    @provide_session
    def test_should_respect_page_size_limit_default(self, session):