# 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': '{}'
        }
    ]
)
```

rule_name = 'rule-{}'.format(short_uid())
        self.events_client.put_rule(Name=rule_name, ScheduleExpression='rate(1 minutes)')
        response = self.events_client.list_rules()
        self.assertEqual(response['Rules'][0]['State'], 'ENABLED')
        response = self.events_client.disable_rule(Name=rule_name)
        response = self.events_client.list_rules(NamePrefix=rule_name)
        self.assertEqual(response['Rules'][0]['State'], 'DISABLED')
        
        self.events_client.delete_rule(Name=rule_name, Force=True)
    def test_scheduled_expression_events(self):
        class HttpEndpointListener(ProxyListener):
            def forward_request(self, method, path, data, headers):
                event = json.loads(to_str(data))
                events.append(event)
                return 200
        local_port = get_free_tcp_port()
        proxy = start_proxy(local_port, backend_url=None, update_listener=HttpEndpointListener())
        wait_for_port_open(local_port)
        topic_name = 'topic-{}'.format(short_uid())