Add support for Null Equality Joins

Join with an equality condition which allows null=null fails. For example, if we use some of this queries:

select ... FROM t1, t2 WHERE t1.c1 = t2.c2 OR (t1.c1 IS NULL AND t2.c2 IS NULL);

select ... FROM t1 INNER JOIN  t2 ON  t1.c1 = t2.c2 OR (t1.c1 IS NULL AND t2.c2 IS NULL);

we got "UNSUPPORTED_OPERATION ERROR". We should add support for this option.