with my_jsonb_table as (
  select unnest(array['[[], [1, 2], [3]]'::jsonb, '[[4, 5], [6], []]'::jsonb]) as val
)
select (select array_agg(values)
        from jsonb_array_elements(my_jsonb_table.val) as elements
        cross join jsonb_array_elements(elements) as values)
from my_jsonb_table;



 array_agg 
-----------
 {1,2,3}
 {4,5,6}
(2 rows)

WITH my_jsonb_table AS (
   SELECT unnest(ARRAY['[[], [1, 2], [3]]'::jsonb, 
                       '[[4, 5], [6], []]'::jsonb]) AS val
)
SELECT val, jsonb_path_query_array(val, '$[*][*]')
  FROM my_jsonb_table;