Created Issue: Datepicker.Valuechanged event is firing for DeviceBack [11614]

__Steps to Reproduce the issue:__
1. Assign Datepicker.Value as null.
1. Run the app and open datepickerdialog.
1. now press device back button.

__Expected behavior:__
datepickerdialog closes but Datepicker.Valuechanged event should not fire.

__Actual behavior:__
datepickerdialog closes and Datepicker.Valuechanged event is firing.