### 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_
if type(conditions) is not list:
        conditions = [conditions]
    if attribute["Type"] == "String.Array":
        values = ast.literal_eval(attribute["Value"])
        for value in values:
            for condition in conditions:
                if evaluate_condition(value, condition, message_attributes, criteria):
                    return True
    else:
        for condition in conditions:
            if evaluate_condition(attribute["Value"], condition, message_attributes, criteria):
                return True
    return False
def check_filter_policy(filter_policy, message_attributes):
    if not filter_policy:
        return True
    for criteria in filter_policy:
        conditions = filter_policy.get(criteria)
        attribute = message_attributes.get(criteria)
        if attribute is None:
            return False
        if (
            evaluate_filter_policy_conditions(conditions, attribute, message_attributes, criteria)
            is False
        ):
            return False
    return True
def is_raw_message_delivery(susbcriber):
    return susbcriber.get("RawMessageDelivery") in ("true", True, "True")