### 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_
LOG.info(
            "Delivering SMS message to %s: %s",
            subscriber["Endpoint"],
            req_data["Message"][0],
        )
        return
    elif subscriber["Protocol"] == "sqs":
        queue_url = None
        try:
            endpoint = subscriber["Endpoint"]
            if "sqs_queue_url" in subscriber:
                queue_url = subscriber.get("sqs_queue_url")
            elif "://" in endpoint:
                queue_url = endpoint
            else:
                queue_name = endpoint.split(":")[5]
                queue_url = aws_stack.get_sqs_queue_url(queue_name)
                subscriber["sqs_queue_url"] = queue_url
            message_group_id = (
                req_data.get("MessageGroupId")[0] if req_data.get("MessageGroupId") else ""
            )
            sqs_client = aws_stack.connect_to_service("sqs")
            
            kwargs = {"MessageGroupId": message_group_id} if SQS_BACKEND_IMPL == "moto" else {}
            sqs_client.send_message(
                QueueUrl=queue_url,
                MessageBody=create_sns_message_body(subscriber, req_data, message_id),
                MessageAttributes=create_sqs_message_attributes(subscriber, message_attributes),
                MessageSystemAttributes=create_sqs_system_attributes(headers),
                **kwargs,