I was able to solve the problem by adding a custom route. Although i had to do the same process in php by first querying all available terms of the type and then exclude them from the query.
$available_terms = array_map( function ( $term ) {
return $term->term_id;
}, get_terms( 'lagerboxen_kategorie' ) );
return get_posts( [
'post_type' => 'lagerboxen',
'tax_query' => [
'relation' => 'AND',
[
'taxonomy' => 'lagerboxen_kategorie',
'field' => 'term_id',
'terms' => $available_terms,
'operator' => 'NOT IN'
]
]
] );
Of course it is still interesting if there is a better way 🙂