### 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)

assert len(notifications) >= 1
            
            execution_arn = executions[0]["executionArn"]
            _execution_input = aws_client.stepfunctions.describe_execution(
                executionArn=execution_arn
            )["input"]
            all_msgs = []
            
            for url in q_urls:
                msgs = aws_client.sqs.receive_message(QueueUrl=url).get("Messages", [])
                assert len(msgs) >= 1
                all_msgs.append(msgs[0])
            return _execution_input, notifications[0], all_msgs
        execution_input, notification, msgs_received = retry(
            received, retries=5, sleep=15, q_urls=[queue_url, fifo_queue_url]
        )
        assert json.loads(notification) == event
        assert json.loads(execution_input) == event
        for msg_received in msgs_received:
            assert json.loads(msg_received["Body"]) == event
        
        target_ids = [topic_target_id, sm_target_id, queue_target_id, fifo_queue_target_id]
        clean_up(rule_name=rule_name, target_ids=target_ids, queue_url=queue_url)
        aws_client.stepfunctions.delete_state_machine(stateMachineArn=state_machine_arn)
    @pytest.mark.parametrize("auth", API_DESTINATION_AUTHS)
    @markers.aws.unknown
    def test_api_destinations(self, httpserver: HTTPServer, auth, aws_client, clean_up):
        token = short_uid()
        bearer = f"Bearer {token}"
        def _handler(_request: Request):