### 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_
"Type": "Notification",
            "MessageAttributes": {
                "attr1": {
                    "Type": "String",
                    "Value": "value1",
                },
                "attr2": {
                    "Type": "String",
                    "Value": "value2",
                },
            },
        }
        self.assertEqual(msg, result)
    def test_create_sns_message_body_json_structure(self):
        action = {
            "Message": ['{"default": {"message": "abc"}}'],
            "MessageStructure": ["json"],
        }
        result_str = sns_listener.create_sns_message_body(self.subscriber, action)
        result = json.loads(result_str)
        self.assertEqual({"message": "abc"}, result["Message"])
    def test_create_sns_message_body_json_structure_raw_delivery(self):
        self.subscriber["RawMessageDelivery"] = "true"
        action = {
            "Message": ['{"default": {"message": "abc"}}'],
            "MessageStructure": ["json"],
        }
        result = sns_listener.create_sns_message_body(self.subscriber, action)
        self.assertEqual({"message": "abc"}, result)
    def test_create_sns_message_body_json_structure_without_default_key(self):