### 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_
self.subscriber["RawMessageDelivery"] = "true"
        action = {
            "Message": ["msg"],
            "Subject": ["subject"],
            "MessageAttributes.entry.1.Name": ["attr1"],
            "MessageAttributes.entry.1.Value.DataType": ["String"],
            "MessageAttributes.entry.1.Value.StringValue": ["value1"],
            "MessageAttributes.entry.2.Name": ["attr2"],
            "MessageAttributes.entry.2.Value.DataType": ["Binary"],
            
            "MessageAttributes.entry.2.Value.BinaryValue": [
                base64.b64encode("value2".encode("utf-8"))
            ],
            "MessageAttributes.entry.3.Name": ["attr3"],
            "MessageAttributes.entry.3.Value.DataType": ["Number"],
            "MessageAttributes.entry.3.Value.StringValue": ["3"],
        }
        attributes = sns_listener.get_message_attributes(action)
        result = sns_listener.create_sqs_message_attributes(self.subscriber, attributes)
        self.assertEqual("String", result["attr1"]["DataType"])
        self.assertEqual("value1", result["attr1"]["StringValue"])
        self.assertEqual("Binary", result["attr2"]["DataType"])
        self.assertEqual("value2".encode("utf-8"), result["attr2"]["BinaryValue"])
        self.assertEqual("Number", result["attr3"]["DataType"])
        self.assertEqual("3", result["attr3"]["StringValue"])
    def test_create_sns_message_timestamp_millis(self):