### 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_
response = Response()
    response._content = """<ErrorResponse xmlns="http://sns.amazonaws.com/doc/2010-03-31/"><Error>
        <Type>Sender</Type>
        <Code>{code_string}</Code>
        <Message>{message}</Message>
        </Error><RequestId>{req_id}</RequestId>
        </ErrorResponse>""".format(
        message=message, code_string=code_string, req_id=short_uid()
    )
    response.status_code = code
    return response
def create_sns_message_body(subscriber, req_data, message_id=None):
    message = req_data["Message"][0]
    protocol = subscriber["Protocol"]
    if six.PY2 and type(message).__name__ == "unicode":
        
        message = message.encode("raw-unicode-escape")
    if req_data.get("MessageStructure") == ["json"]:
        message = json.loads(message)
        try:
            message = message.get(protocol, message["default"])
        except KeyError:
            raise Exception("Unable to find 'default' key in message payload")
    if is_raw_message_delivery(subscriber):
        return message
    data = {
        "Type": req_data.get("Type", ["Notification"])[0],
        "MessageId": message_id,
        "TopicArn": subscriber["TopicArn"],
        "Message": message,