Fix Request #67453 Allow to unserialize empty data.

SplDoublyLinkedList, SplObjectStorage and ArrayObject have empty
constructor (no arg), so it make sense to allow to unserialize empty
data.

This allow the hack (used in various place, including PHPUnit) to
instanciate class without call to constructor to work.
NEWS
Merge branch 'PHP-5.6'

* PHP-5.6:
  NEWS
  Fix Request #67453 Allow to unserialize empty data.