improve not_null and null functions

[bschaatsbergen]

Fixes #76
Fixes #71

• Currently, null checks whether the DynamicValue represents a literal null with no known type. This behavior conflicted with cross-object validation, where the type is known, and the underlying value needs to be checked to see if it evaluates to null. 9725ed fixes this.
• Similarly, for not_null, we’re enabling cross-object validation by first determining if we’re dealing with a literal null value. If not, the underlying value is then checked to confirm whether it is null or not.