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

queue_name = 'queue-{}'.format(short_uid())
        fifo_queue_name = 'queue-{}.fifo'.format(short_uid())
        rule_name = 'rule-{}'.format(short_uid())
        endpoint = '{}://{}:{}'.format(get_service_protocol(), config.LOCALSTACK_HOSTNAME, local_port)
        sm_role_arn = aws_stack.role_arn('sfn_role')
        sm_name = 'state-machine-{}'.format(short_uid())
        topic_target_id = 'target-{}'.format(short_uid())
        sm_target_id = 'target-{}'.format(short_uid())
        queue_target_id = 'target-{}'.format(short_uid())
        fifo_queue_target_id = 'target-{}'.format(short_uid())
        events = []
        state_machine_definition = """
        {
            "StartAt": "Hello",
            "States": {
                "Hello": {
                    "Type": "Pass",
                    "Result": "World",
                    "End": true
                }
            }
        }
        """
        state_machine_arn = self.sfn_client.create_state_machine(
            name=sm_name, definition=state_machine_definition, roleArn=sm_role_arn)['stateMachineArn']
        topic_arn = self.sns_client.create_topic(Name=topic_name)['TopicArn']
        self.sns_client.subscribe(TopicArn=topic_arn, Protocol='http', Endpoint=endpoint)
        queue_url = self.sqs_client.create_queue(QueueName=queue_name)['QueueUrl']
        fifo_queue_url = self.sqs_client.create_queue(
            QueueName=fifo_queue_name, Attributes={'FifoQueue': 'true'})['QueueUrl']