### 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_
"Timestamp": timestamp_millis(),
        "SignatureVersion": "1",
        
        
        "Signature": "EXAMPLEpH+..",
        "SigningCertURL": "https://sns.us-east-1.amazonaws.com/SimpleNotificationService-0000000000000000000000.pem",
    }
    for key in ["Subject", "SubscribeURL", "Token"]:
        if req_data.get(key):
            data[key] = req_data[key][0]
    for key in HTTP_SUBSCRIPTION_ATTRIBUTES:
        if key in subscriber:
            data[key] = subscriber[key]
    attributes = get_message_attributes(req_data)
    if attributes:
        data["MessageAttributes"] = attributes
    return json.dumps(data)
def create_sqs_message_attributes(subscriber, attributes):
    if not is_raw_message_delivery(subscriber):
        return {}
    message_attributes = {}
    for key, value in attributes.items():
        attribute = {"DataType": value["Type"]}
        if value["Type"] == "Binary":
            attribute["BinaryValue"] = base64.decodebytes(to_bytes(value["Value"]))
        else:
            attribute["StringValue"] = str(value.get("Value", ""))
        message_attributes[key] = attribute
    return message_attributes
def get_message_attributes(req_data):