### 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)

def _clean_up(
            bus_name=None,
            rule_name=None,
            target_ids=None,
            queue_url=None,
            log_group_name=None,
        ):
            events_client = aws_client.events
            kwargs = {"EventBusName": bus_name} if bus_name else {}
            if target_ids:
                target_ids = target_ids if isinstance(target_ids, list) else [target_ids]
                events_client.remove_targets(Rule=rule_name, Ids=target_ids, Force=True, **kwargs)
            if rule_name:
                events_client.delete_rule(Name=rule_name, Force=True, **kwargs)
            if bus_name:
                events_client.delete_event_bus(Name=bus_name)
            if queue_url:
                sqs_client = aws_client.sqs
                sqs_client.delete_queue(QueueUrl=queue_url)
            if log_group_name:
                logs_client = aws_client.logs
                log_streams = logs_client.describe_log_streams(logGroupName=log_group_name)
                for log_stream in log_streams["logStreams"]:
                    logs_client.delete_log_stream(
                        logGroupName=log_group_name, logStreamName=log_stream["logStreamName"]
                    )
                logs_client.delete_log_group(logGroupName=log_group_name)
        yield _clean_up
    @markers.aws.validated
    def test_put_target_id_validation(