### 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)

aws_client.events.put_targets(
            Rule=default_bus_rule_name,
            Targets=[{"Id": default_bus_target_id, "Arn": queue_arn}],
        )
        
        entries = [
            {
                "Source": "MySource",
                "DetailType": "CustomType",
                "Detail": json.dumps({"message": "for the default event bus"}),
                "Time": datetime(year=2022, day=1, month=1),
            }
        ]
        response = aws_client.events.put_events(Entries=entries)
        snapshot.match("put-events", response)
        def _get_sqs_messages():
            resp = aws_client.sqs.receive_message(
                QueueUrl=queue_url, VisibilityTimeout=0, WaitTimeSeconds=1
            )
            msgs = resp.get("Messages")
            assert len(msgs) == 1
            aws_client.sqs.delete_message(
                QueueUrl=queue_url, ReceiptHandle=msgs[0]["ReceiptHandle"]
            )
            return msgs
        messages = retry(_get_sqs_messages, retries=5, sleep=0.1)
        snapshot.match("get-events", messages)
        message_body = json.loads(messages[0]["Body"])
        assert message_body["time"] == "2022-01-01T00:00:00Z"