[Proposal] Use isEmpty for the date validator's allowBlank alike the other validators

Heya!

I tried to follow the discussion from which birthed the current date validator, but I couldn't find an answer: why do you not allow empty strings `''` in this validator? The other validators from this addon do so; ember-validators, from where this addon takes a lot, do so ([cf.](https://github.com/offirgolan/ember-validators/blob/1568f472eb5d9851222b9944521ff1ba529641dd/addon/date.js#L29)).

Was there a reason? Maybe it's due to moment.js?

I see you added that in the tests as invalid cases; meanwhile all the other validators have the empty string as, _at minima_, the only "blank" value. Shouldn't we here base the "blank" check on either the `isEmpty()` or `isBlank()` Ember's utils' methods (just as ember-validators, which is used for this addon's number validator)?

To have the same behaviour for such options on all validators would ease the use of the API, I think.