Feat: Support Full Join when HashJoin cannot be used

[KKould]

Feature Request

statement ok
CREATE TABLE onecolumn (id INT PRIMARY KEY, x INT NULL)

statement ok
CREATE TABLE empty (e_id INT PRIMARY KEY, x INT)

statement ok
INSERT INTO onecolumn(id, x) VALUES (0, 44), (1, NULL), (2, 42)

# TODO: Full Join on nested loop join
query II
SELECT * FROM onecolumn AS a(x) FULL OUTER JOIN empty AS b(y) ON a.x = b.y ORDER BY a.x
----
42 NULL
44 NULL
NULL NULL