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 add a small documentation comment, even if it's deadly simple and for internal purpose only, better doing it now than never. And explaining the rationale is probably the most important.

Here something like:
> Returns `null` in case the value is considered _void_, which means `undefined` or already `null`. Useful for contexts using strict equality comparison but in which we want to merge the meaning of `null` and `undefined`.