### 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_rule(
            Name="ScheduledLambda", ScheduleExpression="rate(1 minute)"
        )  
        cleanups.append(lambda: aws_client.events.delete_rule(Name="ScheduledLambda"))
        aws_client.events.put_targets(
            Rule="ScheduledLambda", Targets=[{"Id": "calllambda1", "Arn": fn_arn}]
        )
        cleanups.append(
            lambda: aws_client.events.remove_targets(Rule="ScheduledLambda", Ids=["calllambda1"])
        )
        aws_client.events.put_events(
            Entries=[
                {
                    "Source": "MySource",
                    "DetailType": "CustomType",
                    "Detail": json.dumps({"message": "manually invoked"}),
                }
            ]
        )
        def check_invocation():
            events_after = aws_client.logs.filter_log_events(logGroupName=f"/aws/lambda/{fn_name}")
            
            assert len([e for e in events_after["events"] if "START" in e["message"]]) >= 1
            assert (
                len([e for e in events_after["events"] if "manually invoked" in e["message"]]) == 0
            )
        retry(check_invocation, sleep=5, retries=15)
    @markers.aws.validated
    def test_put_events_nonexistent_event_bus(