### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

When a scheduled rules runs, an empty event is generated and sent to targets.

### Expected Behavior

An event like this sample should be sent.
```json
{
  "version": "0",
  "id": "89d1a02d-5ec7-412e-82f5-13505f849b41",
  "detail-type": "Scheduled Event",
  "source": "aws.events",
  "account": "123456789012",
  "time": "2016-12-30T18:44:49Z",
  "region": "us-east-1",
  "resources": ["arn:aws:events:us-east-1:123456789012:rule/SampleRule"],
  "detail": {}
}
```

### How are you starting LocalStack?

With a docker-compose file

### Steps To Reproduce

#### How are you starting localstack (e.g., `bin/localstack` command, arguments, or `docker-compose.yml`)

    docker-compose up

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

    awslocal events put-rule --name scheduled_rule --schedule-expression "rate(1 minutes)"
    awslocal events put-targets --rule scheduled_rule --targets "Id"="1","Arn"="arn:aws:logs:us-east-1:000000000000:log-group:/aws/events/echo"
    awslocal--endpoint-url=http://localhost:4566 logs tail "/aws/events/echo"

### Environment

```markdown
- OS: macOS Monterey
- LocalStack: 0.13.3
```


### Anything else?
I think [this](https://github.com/localstack/localstack/blob/1567f25a8f672f4dec23cb7cb1aadb5a744366f8/localstack/services/events/events_listener.py#L92) means an non-empty event is sent only when an input is `Constant (JSON text)`.
![image](https://user-images.githubusercontent.com/4127030/151643813-a4d1e113-dba3-41ad-8400-da57e4f674a1.png)

"target-header": "target_header_value",
                            "overwritten_header": "changed",
                        },
                        "QueryStringParameters": {
                            "target_query": "t_query",
                            "overwritten_query": "changed",
                        },
                    },
                }
            ],
        )
        entries = [
            {
                "Source": "source-123",
                "DetailType": "type-123",
                "Detail": '{"i": 0}',
            }
        ]
        aws_client.events.put_events(Entries=entries)
        
        aws_client.events.delete_connection(Name=connection_name)
        aws_client.events.delete_api_destination(Name=dest_name)
        clean_up(rule_name=rule_name, target_ids=target_id)
        to_recv = 2 if auth["type"] == "OAUTH_CLIENT_CREDENTIALS" else 1
        poll_condition(lambda: len(httpserver.log) >= to_recv, timeout=5)
        event_request, _ = httpserver.log[-1]
        event = event_request.get_json(force=True)
        headers = event_request.headers
        query_args = event_request.args