Lightning scheduler does not correctly validate date.

This patch will fix the issue for 8.x-3.19.

The last submitted patch, 3: 3366897-3.patch, failed testing. View results →

Thanks @shailja179,

I gave this a test, but unfortunately this patch doesn't solve the issue.

- It's still possible to enter and save an invalid date in the scheduled transition form.
- The invalid transition date is still recorded in the `node__scheduled_transition_date` table.
- Running drupal cron with an invalid date in `node__scheduled_transition_date` table still causes an error that blocks the entire cron run:
`Error: Call to a member function getTimestamp() on null in Drupal\lightning_scheduler\TransitionSet->toArray() (line 60...`