SMW\DISerializer array_map, callback and null value

Now that the callback contains two arrays and it is now also called
even $resultArray->getContent() is empty which didn't
meant much before since it would return an empty array() but now an
empty $resultArray->getContent() will cause a null instance
when calling getSerialization() therefore we explicitly check
whether $resultArray->getContent() is empty or not and for for all
other cases we return an array() to ensure all elements have the same
length.

Argument 1 passed to SMW\DISerializer::getSerialization()
must be an instance of SMWDataItem, null given in
... DISerializer.php on line 48

Change-Id: I66f93d4f98017fd8b9682907b2a8768c568c3690