Support flattening of empty-FROM subqueries and one-row VALUES tables.
We can't handle this in the general case due to limitations of the planner's data representations; but we can allow it in many useful cases, by being careful to flatten only when we are pulling a single-row subquery up into a FROM (or, equivalently, inner JOIN) node that will still have at least one remaining relation child. Per discussion of an example from Kyotaro Horiguchi.
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