### 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)

self, sqs_create_queue, events_put_rule, snapshot, aws_client
    ):
        rule_name = f"rule-{short_uid()}"
        queue_url = sqs_create_queue()
        queue_arn = self._get_queue_arn(queue_url, aws_client.sqs)
        events_put_rule(
            Name=rule_name, EventPattern=json.dumps(TEST_EVENT_PATTERN), State="ENABLED"
        )
        target_id = "!@#$@!#$"
        with pytest.raises(ClientError) as e:
            aws_client.events.put_targets(
                Rule=rule_name,
                Targets=[
                    {"Id": target_id, "Arn": queue_arn, "InputPath": "$.detail"},
                ],
            )
        snapshot.add_transformer(snapshot.transform.regex(target_id, "invalid-target-id"))
        snapshot.match("error", e.value.response)
        target_id = f"{long_uid()}-{long_uid()}-extra"
        with pytest.raises(ClientError) as e:
            aws_client.events.put_targets(
                Rule=rule_name,
                Targets=[
                    {"Id": target_id, "Arn": queue_arn, "InputPath": "$.detail"},
                ],