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
Not completely related to the PR, but the JSDoc should be updated, and corrected at the same time (_overrided_ -> _overridden_, _equals_ -> _equal_, too specific description while relying on a simple other method, parameter name mismatch). It should indicate what "considering as equal" means: here having the two values `null` or `undefined` makes them _as equal_ in addition to strict equality.