### 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 == "ListTagsForResource":
                tags = do_list_tags_for_resource(topic_arn)
                content = "<Tags/>"
                if len(tags) > 0:
                    content = "<Tags>"
                    for tag in tags:
                        content += "<member>"
                        content += "<Key>%s</Key>" % tag["Key"]
                        content += "<Value>%s</Value>" % tag["Value"]
                        content += "</member>"
                    content += "</Tags>"
                return make_response(req_action, content=content)
            elif req_action == "CreateTopic":
                sns_backend = SNSBackend.get()
                topic_arn = aws_stack.sns_topic_arn(req_data["Name"][0])
                tag_resource_success = self._extract_tags(topic_arn, req_data, True, sns_backend)
                sns_backend.sns_subscriptions[topic_arn] = (
                    sns_backend.sns_subscriptions.get(topic_arn) or []
                )
                
                if not tag_resource_success:
                    return make_error(
                        code=400,
                        code_string="InvalidParameter",
                        message="Topic already exists with different tags",
                    )
            elif req_action == "TagResource":
                sns_backend = SNSBackend.get()
                self._extract_tags(topic_arn, req_data, False, sns_backend)
                return make_response(req_action)