### 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_
if method == "OPTIONS":
            return 200
        
        try:
            aws_stack.check_valid_region(headers)
            aws_stack.set_default_region_in_headers(headers)
        except Exception as e:
            return make_error(message=str(e), code=400)
        if method == "POST":
            
            req_data = urlparse.parse_qs(to_str(data), keep_blank_values=True)
            
            if not req_data:
                parsed_path = urlparse.urlparse(path)
                req_data = urlparse.parse_qs(parsed_path.query, keep_blank_values=True)
            req_action = req_data["Action"][0]
            topic_arn = (
                req_data.get("TargetArn") or req_data.get("TopicArn") or req_data.get("ResourceArn")
            )
            if topic_arn:
                topic_arn = topic_arn[0]
                topic_arn = aws_stack.fix_account_id_in_arns(topic_arn)
            if req_action == "SetSubscriptionAttributes":
                sub = get_subscription_by_arn(req_data["SubscriptionArn"][0])
                if not sub:
                    return make_error(message="Unable to find subscription for given ARN", code=400)
                attr_name = req_data["AttributeName"][0]
                attr_value = req_data["AttributeValue"][0]
                sub[attr_name] = attr_value
                return make_response(req_action)