function set_interests_null( $update_data, $user_id, $contact_id, $form_id ) {

    if ( 2 == $form_id && empty( $update_data['interests'] ) ) {
        $update_data['interests'] = null;
    }

    return $update_data;

}

add_filter( 'wpf_forms_pre_submission', 'set_interests_null', 10, 4 );