WITH Clause in SELECT with empty result set

I just found a workaround by doing a UNION with DUAL, but it requires defining a specific value for each column in the query.  It would still be nice to not have to do this.

Actually, the UNION with DUAL will only address the case where one of the *with_clauses* return an empy result set.  But if the with_clause has values, I'm not gettingn the same result set as running the whole query vs. running the "select for the with_clause UNION DUAL".

FROM with_clause_1, with_clause_2

WITH set1 AS(SELECT * FROM Customer),
     set2 AS(SELECT * FROM Customer)
SELECT * FROM set1
     UNION ALL
SELECT * FROM set2;

daniel=> \e
 id | name |  birthday  
----+------+------------
  1 | Dan  | 1983-01-08
  2 | Ian  | 1984-01-09
  3 | Lee  | 1985-02-10
  1 | Dan  | 1983-01-08
  2 | Ian  | 1984-01-09
  3 | Lee  | 1985-02-10
(6 rows)