Added Null validator, validators now handle null/undefined, 100% coverage
For properties/values that are expected to be `null`, the `valNull` validator will now handle it. Validators must now also handle `null` and `undefined` values according to the specified qualifier, returning success if the given value is one or the other, as long as the qualifier permits those values. For example, if the qualifier is EXPECTED and the value is `null`, then any validator would return success for that value. Given the same qualifier, but a value of `undefined`, all validators (except for the ANY type which is somewhat of an exception) would return an error because that value is not permitted given that qualifier. To help with `null` and `undefined` values, a new `nilPermitted()` method is provided by the `qualifiers` module. Finally, test coverage is back to 100%!!
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