Right and outer join skip first null element

[sppmacd]

Reproduce:

1. Checkout the pr Core: Entirely refactor subquery system #4
2. Add --display to RIGHT JOIN and OUTER JOIN statements in select/joins/simple_joins.sql
3. Run that test
4. See that elements with id=6 are skipped, which is noted as a FIXME here.

Example:

SELECT * FROM tablea FULL OUTER JOIN tableb ON tablea.id = tableb.id;

gives