### 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_
def patch_create_platform_endpoint(self, *args):
        try:
            return create_platform_endpoint_orig(self, *args)
        except DuplicateSnsEndpointError:
            custom_user_data, token = args[2], args[3]
            for endpoint in self.platform_endpoints.values():
                if endpoint.token == token:
                    if custom_user_data and custom_user_data != endpoint.custom_user_data:
                        raise DuplicateSnsEndpointError(
                            "Endpoint already exist for token: %s with different attributes" % token
                        )
                    return endpoint
    create_platform_endpoint_orig = MotoSNSBackend.create_platform_endpoint
    MotoSNSBackend.create_platform_endpoint = patch_create_platform_endpoint
patch_moto()
UPDATE_SNS = ProxyListenerSNS()
def unsubscribe_sqs_queue(queue_url):
    """Called upon deletion of an SQS queue, to remove the queue from subscriptions"""
    sns_backend = SNSBackend.get()
    for topic_arn, subscriptions in sns_backend.sns_subscriptions.items():
        subscriptions = sns_backend.sns_subscriptions.get(topic_arn, [])
        for subscriber in list(subscriptions):
            sub_url = subscriber.get("sqs_queue_url") or subscriber["Endpoint"]
            if queue_url == sub_url:
                subscriptions.remove(subscriber)
def message_to_subscribers(