### 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_
"""Fix account ID in request payload. All external-facing responses contain our
        predefined account ID (defaults to 000000000000), whereas the backend endpoint
        from moto expects a different hardcoded account ID (123456789012)."""
        return aws_stack.fix_account_id_in_arns(
            data,
            colon_delimiter="%3A",
            existing=TEST_AWS_ACCOUNT_ID,
            replace=MOTO_ACCOUNT_ID,
        )
    def return_response(self, method, path, data, headers, response):
        
        super(ProxyListenerSNS, self).return_response(method, path, data, headers, response)
        if method == "POST" and path == "/":
            
            data = aws_stack.fix_account_id_in_arns(data, colon_delimiter="%3A")
            aws_stack.fix_account_id_in_arns(response)
            
            search = r"<entry><key>[^<]+</key>\s*<value>\s*None\s*</[^>]+>\s*</entry>"
            response_regex_replace(response, search, "")
            
            req_data = urlparse.parse_qs(to_str(data))
            req_action = req_data["Action"][0]
            if req_action == "Subscribe" and response.status_code < 400:
                response_data = xmltodict.parse(response.content)
                topic_arn = (req_data.get("TargetArn") or req_data.get("TopicArn"))[0]
                filter_policy = (req_data.get("FilterPolicy") or [None])[0]
                attributes = get_subscribe_attributes(req_data)
                sub_arn = response_data["SubscribeResponse"]["SubscribeResult"]["SubscriptionArn"]
                do_subscribe(