Set value to `null` when no date is selected due clearing the input
Set value to `null` when no date is selected due clearing the input

No tests for the time being due to weird Pikaday timing issues.

helm-inc/ember-pikaday-time,edgycircle/ember-pikaday,jeffreybiles/ember-pikaday,fasteragile/ember-pikaday,digitallumens/ember-pikaday,Inkstone-Inc/ember-pikaday-time,joshuairl/ember-pikaday-with-time,Inkstone-Inc/ember-pikaday-time,helm-inc/ember-pikaday-time,leizhao4/ember-pikaday,leizhao4/ember-pikaday,GabKlein/ember-pikaday,LevelbossMike/ember-pikaday,fasteragile/ember-pikaday,jeffreybiles/ember-pikaday,GabKlein/ember-pikaday,LevelbossMike/ember-pikaday,joshuairl/ember-pikaday-with-time,edgycircle/ember-pikaday,digitallumens/ember-pikaday