### 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_
sns_backend = SNSBackend.get()
    sns_backend.sns_tags[topic_arn] = [t for t in _get_tags(topic_arn) if t["Key"] not in tag_keys]
def get_subscription_by_arn(sub_arn):
    sns_backend = SNSBackend.get()
    
    for key, subscriptions in sns_backend.sns_subscriptions.items():
        for sub in subscriptions:
            if sub["SubscriptionArn"] == sub_arn:
                return sub
def make_response(op_name, content="", message_id=None):
    response = Response()
    if not content:
        message_id = message_id or str(uuid.uuid4())
        content = "<MessageId>%s</MessageId>" % message_id
    response._content = """<{op_name}Response xmlns="http://sns.amazonaws.com/doc/2010-03-31/">
        <{op_name}Result>
            {content}
        </{op_name}Result>
        <ResponseMetadata><RequestId>{req_id}</RequestId></ResponseMetadata>
        </{op_name}Response>""".format(
        op_name=op_name, content=content, req_id=short_uid()
    )
    response.status_code = 200
    return response
def make_error(message, code=400, code_string="InvalidParameter"):