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

self.events_client = aws_stack.connect_to_service('events', Region='eu-west-1')
        response = self.events_client.put_events(
            Entries=[
                {
                    'DetailType': 'Test',
                    'Detail': '{}'
                }
            ]
        )
        self.assertIn('Entries', response)
    def test_put_event_with_content_base_rule_in_pattern(self):
        queue_name = 'queue-{}'.format(short_uid())
        rule_name = 'rule-{}'.format(short_uid())
        target_id = 'target-{}'.format(short_uid())
        sqs_client = aws_stack.connect_to_service('sqs')
        queue_url = sqs_client.create_queue(QueueName=queue_name)['QueueUrl']
        queue_arn = aws_stack.sqs_queue_arn(queue_name)
        pattern = {
            'Source': [{'exists': True}],
            'detail-type': [{'prefix': 'core.app'}],
            'Detail': json.dumps({
                'decription': ['this-is-event-details'],
                'amount': [200],
                'salary': [2000, 4000],
                'env': ['dev', 'prod'],