### 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_
elif req_action == "UntagResource":
                tags_to_remove = []
                req_tags = {k: v for k, v in req_data.items() if k.startswith("TagKeys.member.")}
                req_tags = req_tags.values()
                for tag in req_tags:
                    tags_to_remove.append(tag[0])
                do_untag_resource(topic_arn, tags_to_remove)
                return make_response(req_action)
            data = self._reset_account_id(data)
            return Request(data=data, headers=headers, method=method)
        return True
    @staticmethod
    def _extract_tags(topic_arn, req_data, is_create_topic_request, sns_backend):
        tags = []
        req_tags = {k: v for k, v in req_data.items() if k.startswith("Tags.member.")}
        existing_tags = sns_backend.sns_tags.get(topic_arn, None)
        
        for i in range(int(len(req_tags.keys()) / 2)):
            key = req_tags["Tags.member." + str(i + 1) + ".Key"][0]
            value = req_tags["Tags.member." + str(i + 1) + ".Value"][0]
            tag = {"Key": key, "Value": value}
            tags.append(tag)
            
            
            if is_create_topic_request and existing_tags is not None and tag not in existing_tags:
                return False
        do_tag_resource(topic_arn, tags)
        return True
    @staticmethod
    def _reset_account_id(data):