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