TYPO3 TCA select, NULL in an array of elements

I did an extension in Typo3 4.5 with extbase and fluid. Now, to insert some data, I use the "back" module of the backend module, which creates some forms with TCA tables. To make the selection box optional, I insert an element in front of the external table as follows:

    'feuser' => array(
        'exclude' => 0,
        'label' => 'LLL:EXT:yes/Resources/Private/Language/locallang_db.xml:tx_yes_domain_model_schools.feuser',
        'config' => array(
            'type' => 'select',
            'items' => array(
                array('', NULL),
            ),
            'foreign_table' => 'fe_users',
            'maxitems' => 1,
        ),
    ),

Now, since I have a relationship (with the addition of NULL) in my database, I need to insert a NULL value. But it does not seem to work. I also tried "," "and" 0. But this also does not work. "

I would be grateful for any help.