[TYPO3-mvc] Query: Find items with empty ObjectStorage

[Wild Axel]

Hello,
I'm trying to search for items with an empty ObjectStorage (attribute: category).

Something like this:

$query = $this->createQuery();
$constraints[] = $query->equals('category', ??);
..
$query->matching($query->logicalAnd($constraints));                
return $query->execute();

I tried different option (contains, in, equals with '', NULL)
but could not find the solution.

using $query->isEmtpy('category') displays an error:
"sEmpty is not supported by generic persistence"

Can someone help me?

Regards, Axel