The date validator has a different allowBlank validation

Hi! :wave:

While working with [ember-changeset-validations](https://github.com/poteto/ember-changeset-validations), which uses this addon quite extensively, I saw there was an "inconsistency" in the behaviour of the date validator's `allowBlank` option, which checks nullish values and empty strings instead of using Ember's util `isEmpty`, as per all other validators ([cf.](https://github.com/offirgolan/ember-validators/blob/1568f472eb5d9851222b9944521ff1ba529641dd/addon/date.js#L29)).
I also saw that was introduced when removing Moment ([cf. commit](https://github.com/offirgolan/ember-validators/commit/f829984c8c28152f6e15d0b091308583fb1ac997)). Was it meant to be?