Your best option will be to get an array of term ids by using get_terms
which belongs to the authors
taxonomy and then using that as your array of terms in your tax_query
The following requires PHP5.4+
$term_ids = get_terms( 'authors', ['fields' => 'ids'] ); // Only get term ids
$args = [
'tax_query' => [
[
'taxonomy' => 'authors',
'terms' => $term_ids,
]
],
// Rest of your arguments
];