fix Error tooltip not displayed in a datepicker

It happens when the datepicker is not initialized yet and an invalid value is entered : null is compared to undefined, resetting the DP state
I would even add this utility function as part of http://ariatemplates.com/aria/guide/apps/apidocs/#aria.utils.Type
There could be an equivalent method returning `undefined` instead of `null` (`normalizeUndefined` for instance), or this one could be made more generic and _customizable_ but this might be confusing.