# Type of request: This is a ...

[X ] bug report
[ ] feature request

# Detailed description
Calling PutEvents operation returns 500 if no `Detail` is specified. This case is similar to #3043

## Expected behavior
According to the doc https://docs.aws.amazon.com/eventbridge/latest/APIReference/API_PutEventsRequestEntry.html `Details` is optional. 

```
 Detail
    A valid JSON string. There is no other schema imposed. The JSON string may contain fields and nested subobjects.
    Type: String
    Required: No
```

So this should works and set `Detail` as `{}`

```
eb_client = boto3.client('events', endpoint_url='http://localhost:4587')
eb_client.put_events(
    Entries=[
        {
            'DetailType': 'Test'
        }
    ]
)
```

## Actual behavior
500 is returned

# Steps to reproduce

1. Start LocalStack with SERVICES=events
2. Run the client code

## Command used to start LocalStack

```
docker run --rm --name localstack -p 4587:4587 -e SERVICES=events -e DEBUG=true localstack/localstack
```

## Client code (AWS SDK code snippet, or sequence of "awslocal" commands)

CLI:

```
aws events put-events --endpoint-url http://localhost:4587 --entries '[{"DetailType": "Test"}]'
```

Or via python:

```
eb_client = boto3.client('events', endpoint_url='http://localhost:4587')
eb_client.put_events(
    Entries=[
        {
            'DetailType': 'Test',
            'Detail': '{}'
        }
    ]
)
```

resp = sqs_client.receive_message(QueueUrl=queue_url)
            return resp.get('Messages')
        messages = retry(get_message, retries=3, sleep=1, queue_url=queue_url)
        self.assertEqual(len(messages), 1)
        self.assertEqual(json.loads(messages[0].get('Body')), EVENT_DETAIL)
        messages = retry(get_message, retries=3, sleep=1, queue_url=queue_url_1)
        self.assertEqual(len(messages), 1)
        self.assertEqual(json.loads(messages[0].get('Body')).get('detail'), EVENT_DETAIL)
        self.events_client.put_events(
            Entries=[{
                'EventBusName': bus_name,
                'Source': 'dummySource',
                'DetailType': TEST_EVENT_PATTERN['detail-type'][0],
                'Detail': json.dumps(TEST_EVENT_PATTERN['Detail'][0])
            }]
        )
        messages = retry(get_message, retries=3, sleep=1, queue_url=queue_url)
        self.assertEqual(messages, None)
        
        self.cleanup(bus_name, rule_name, target_id, queue_url=queue_url)
    def test_put_event_without_source(self):