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
* Fixed#1261 dynamodb FilterExpression bugs
FilterExpression was incorrectly handling numbers, stupid typo there. Also >= <= and <> was not being parsed correctly.
* Switched up logic a bit for better end result. Fixes#1263
* Fixed another bug
