present checks ''(empty string) as well as null and undefined#
All validators that validate aginst the value of the argument passed for validation return false for null and undefined
Merge branch 'inconsistent-handling-of-null-undefined' into full-separation-of-mandation-and-value-validity
Whether or not a value is mandatory completely independent of the validity of its value *when and only when* it is provided
Merge pull request #8 from sensemaking/full-separation-of-mandation-and-value-validity

Full separation of mandation and value validity