
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

airflow/models/log.py
"""Used to actively log events to the database"""
    __tablename__ = "log"
    id = Column(Integer, primary_key=True)
    dttm = Column(UtcDateTime)
    dag_id = Column(String(ID_LEN, **COLLATION_ARGS))
    task_id = Column(String(ID_LEN, **COLLATION_ARGS))
    event = Column(String(30))
    execution_date = Column(UtcDateTime)
    owner = Column(String(500))
    extra = Column(Text)
    __table_args__ = (Index('idx_log_dag', dag_id),)
    def __init__(self, event, task_instance, owner=None, extra=None, **kwargs):
        self.dttm = timezone.utcnow()
        self.event = event
        self.extra = extra
        task_owner = None
        if task_instance:
            self.dag_id = task_instance.dag_id
            self.task_id = task_instance.task_id
            self.execution_date = task_instance.execution_date
            task_owner = task_instance.task.owner
        if 'task_id' in kwargs:
            self.task_id = kwargs['task_id']
        if 'dag_id' in kwargs:
            self.dag_id = kwargs['dag_id']
        if 'execution_date' in kwargs:
            if kwargs['execution_date']:
                self.execution_date = kwargs['execution_date']
        self.owner = owner or task_owner