Assertion failure in new outer/semi/anti join code

The following query causes an assertion failure on CVS head:

SELECT * FROM (SELECT 1 AS i) AS a WHERE NOT EXISTS (SELECT 1 WHERE 1 <> i);

TRAP: FailedAssertion("!(!bms_is_empty(min_righthand))", File: "initsplan.c", Line: 685)

Looks like it's assuming there's at least one relation on each side of the
join. Incidentally originally triggered with a VALUES clause but I think by
the point the code runs that distinction is long gone.

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