### 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_
return make_error(
                        message="Token not specified in confirm subscription request",
                        code=400,
                    )
                do_confirm_subscription(req_data.get("TopicArn")[0], req_data.get("Token")[0])
            elif req_action == "Unsubscribe":
                if "SubscriptionArn" not in req_data:
                    return make_error(
                        message="SubscriptionArn not specified in unsubscribe request",
                        code=400,
                    )
                do_unsubscribe(req_data.get("SubscriptionArn")[0])
            elif req_action == "DeleteTopic":
                do_delete_topic(topic_arn)
            elif req_action == "Publish":
                if req_data.get("Subject") == [""]:
                    return make_error(code=400, code_string="InvalidParameter", message="Subject")
                sns_backend = SNSBackend.get()
                
                
                if "PhoneNumber" not in req_data and "TargetArn" not in req_data:
                    if topic_arn not in sns_backend.sns_subscriptions:
                        return make_error(
                            code=404,
                            code_string="NotFound",
                            message="Topic does not exist",
                        )
                message_id = publish_message(topic_arn, req_data, headers)
                
                return make_response(req_action, message_id=message_id)