opt: fix zero-column groups in joins colliding

[justinj]

It's fundamental to the optimizer that any two unrelated (i.e., one is
not the parent of the other) expressions in the tree have disjoint
column sets. Usually, this prevents the same relational expression from
occurring in the tree more than once, but in the case of a group with
zero columns, this is not true.

This allows the following group collision to occur:

Let A be the 0-column values node with two rows VALUES (), (),
and B be the 0-column values node with three rows VALUES (), (), ().

Then consider the following query:

(A JOIN B) UNION (B JOIN A)

During build, we add A JOIN B and B JOIN A to separate memo groups.
Then, during exploration, we apply the CommuteJoin rule to transform
A JOIN B to B JOIN A. This attempts to get interned into the same
group, but the interner finds that B JOIN A already exists in a
different group, and panics.

As a side note, a values node with no columns is not valid SQL, but it
can be constructed via the MergeProjectWithValues rule:

SELECT 1 FROM (VALUES (1))
=> MergeProjectWithValues
SELECT 1 FROM (VALUES ())

This commit fixes the problem by simply disabling these types of
explorations on groups with zero columns. This isn't a perfect fix,
since the subtle problem is still lurking, and people going forward need
to understand its implications, but it will fix the problem for the time
being.

Release note (bug fix): fixed several related panics in the optimizer
related to plan exploration.

[cockroach-teamcity]

This change is 

[andy-kimball]

Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @andy-kimball, @RaduBerinde, and @rytaft)

[justinj]

TFTR!

bors r+

[craig]

Build succeeded

• GitHub CI (Cockroach)