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

This also the hack (used in various place, including PHPUnit) to instanciate class without call to constructor to work. 

Test script:
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$a = new ArrayObject(array());
$a->unserialize("");


Expected result:
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No error

Actual result:
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Fatal error: Uncaught exception 'UnexpectedValueException' with message 'Empty serialized string cannot be empty' in ...