The AWS documentation says that a ComparisonOperator of NULL means
the attribute should not exist, whereas NOT_NULL means that the
attribute should exist. It explicitly says that an attribute with a
value of NULL is considered to exist, which contradicts our previous
implementation. This affects both put_item and get_item in dynamodb2.
https://docs.aws.amazon.com/amazondynamodb/latest/APIReference/API_Condition.html
* Fix path detection for deleting keys in S3 bucket
* Fix stack deletion ensure delete method exists on object
* Previous tests were using a stack with no resources
* Fix DESCRIBE_STACK_RESOURCES_RESPONSE,
* Previously untested code path
