https://codex.wordpress.org/Class_Reference/WP_Query#Custom_Field_Parameters, add a query for “NOT EXISTS”
'meta_query' => array(
'relation' => 'OR',
array(
'key' => 'hidden',
'value' => '1',
'compare' => '!='
),
array(
'key' => 'hidden',
'compare' => 'NOT EXISTS'
)
);
);