This is a backport of the changes in #10799 to 6X_STABLE.

When we have an outer ref in a subquery, like this

select * from foo where foo.a is null or foo.a = (select foo.b from bar)

then we can't simply use the outer reference for the condition when
we unnest the subquery into an apply. This is because if the subquery
returns no rows, then we must be using a NULL instead of the outer
reference.

We have code to handle this for quantified subqueries, but not for
scalar subqueries.

When we translate the generated DXL to a plan, we assert when we find
an outer reference in the project list of a subquery. In rare cases,
we might also crash, when the subquery contained a project with multiple
values below the outer reference (see added test in gporca.sql).

The "fix" (more a workaround) is to force a fallback when we detect
this situation during unnesting of a scalar subquery.