cont number of nulls by rows from a given table Postgres

CREATE OR REPLACE FUNCTION query_builder(tab_name text) RETURNS table ( aaa int ) LANGUAGE plpgsql AS $$
DECLARE	
	built_query varchar;
BEGIN
	 SELECT 'SELECT (
	  cast(f."' || STRING_AGG (column_name, '" IS NULL as int) + cast(f."') || '" IS NULL as int)) as sum_nulls from public."'|| tab_name ||'" as f; ' into built_query
	from information_schema.columns
	where table_name = tab_name
	group by table_name; 
    RETURN query EXECUTE built_query;
END; $$;

SELECT query_builder('table_name')