### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current Behavior

I'm encountering an issue with the "exists" property on SNS Filter Policy. It logs that it can't match the policy even though I would expect it to match.

```
2021-07-28T17:57:02:INFO:localstack.services.sns.sns_listener: SNS filter policy {'source': ['source1'], 'priority': [{'exists': False}]} 
does not match attributes {'clientId': {'Type': 'String', 'Value': 'Id123'}, 'source': {'Type': 'String', 'Value': 'source1'}, 'awsRegionName': {'Type': 'String', 'Value': 'us-east-1'}}
```





### Expected Behavior

Policy matches and message is sent to subscribers.

### 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`)

    I run in a docker-compose script and load my CloudFormation script on init.

Using the following CloudFormation script I create the subscription. 

```
"Subscription": {
            "Type": "AWS::SNS::Subscription",
            "Properties": {
                "TopicArn": {
                    "Ref": "MyTopic"
                },
                "Endpoint": {
                    "Fn::GetAtt": [
                        "MyQueue",
                        "Arn"
                    ]
                },
                "Protocol": "sqs",
                "FilterPolicy": {
                    "source": [
                        "source1"
                    ],
                    "priority":  [{"exists": false}]
                },
                "RawMessageDelivery": true
            }
        },
```



### Environment

```markdown
- OS: Windows
- LocalStack: latest
```


### Anything else?

_No response_
data=message_body,
                verify=False,
            )
            response.raise_for_status()
        except Exception as exc:
            LOG.info(
                "Received error on sending SNS message, putting to DLQ (if configured): %s" % exc
            )
            sns_error_to_dead_letter_queue(subscriber["SubscriptionArn"], req_data, str(exc))
        return
    elif subscriber["Protocol"] == "application":
        try:
            sns_client = aws_stack.connect_to_service("sns")
            sns_client.publish(TargetArn=subscriber["Endpoint"], Message=message)
        except Exception as exc:
            LOG.warning(
                "Unable to forward SNS message to SNS platform app: %s %s"
                % (exc, traceback.format_exc())
            )
            sns_error_to_dead_letter_queue(subscriber["SubscriptionArn"], req_data, str(exc))
        return
    elif subscriber["Protocol"] == "email":
        ses_client = aws_stack.connect_to_service("ses")
        if subscriber.get("Endpoint"):
            ses_client.verify_email_address(EmailAddress=subscriber.get("Endpoint"))
            ses_client.verify_email_address(EmailAddress="admin@localstack.com")
            ses_client.send_email(
                Source="admin@localstack.com",
                Message={
                    "Body": {"Text": {"Data": message}},