### 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_
"Subject": {"Data": "SNS-Subscriber-Endpoint"},
                },
                Destination={"ToAddresses": [subscriber.get("Endpoint")]},
            )
    else:
        LOG.warning('Unexpected protocol "%s" for SNS subscription' % subscriber["Protocol"])
def publish_message(topic_arn, req_data, headers, subscription_arn=None, skip_checks=False):
    sns_backend = SNSBackend.get()
    message = req_data["Message"][0]
    message_id = str(uuid.uuid4())
    if topic_arn and ":endpoint/" in topic_arn:
        
        cache = sns_backend.platform_endpoint_messages[topic_arn] = (
            sns_backend.platform_endpoint_messages.get(topic_arn) or []
        )
        cache.append(req_data)
    LOG.debug("Publishing message to TopicArn: %s | Message: %s" % (topic_arn, message))
    start_thread(
        lambda _: message_to_subscribers(
            message_id,
            message,
            topic_arn,
            req_data,
            headers,
            subscription_arn,
            skip_checks,
        )
    )
    return message_id
def do_delete_topic(topic_arn):