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

import os
import json
import uuid
import unittest
from localstack import config
from localstack.utils import testutil
from localstack.utils.aws import aws_stack
from localstack.utils.common import (
    load_file, retry, short_uid, get_free_tcp_port, wait_for_port_open, to_str, get_service_protocol
)
from localstack.services.infra import start_proxy
from localstack.utils.testutil import check_expected_lambda_log_events_length
from localstack.services.generic_proxy import ProxyListener
from localstack.services.events.events_listener import EVENTS_TMP_DIR
from localstack.services.awslambda.lambda_utils import LAMBDA_RUNTIME_PYTHON36
THIS_FOLDER = os.path.dirname(os.path.realpath(__file__))
TEST_EVENT_BUS_NAME = 'command-bus-dev'
EVENT_DETAIL = '{\"command\":\"update-account\",\"payload\":{\"acc_id\":\"0a787ecb-4015\",\"sf_id\":\"baz\"}}'
TEST_EVENT_PATTERN = {
    'Source': ['core.update-account-command'],
    'detail-type': ['core.update-account-command'],
    'Detail': [EVENT_DETAIL]
}
class EventsTest(unittest.TestCase):
    def setUp(self):