The couldHaveUnknownBlockPlaceholder helper was added to detect when a
set block has a placeholder for an unknown number of values. This worked
fine when the number increased from 1, but we were still attempting to
validate the unknown placeholder against the empty set when the final
count turned out to be 0.
Since we can't differentiate the unknown dynamic placeholder value from
an actual set value, we have to skip that object's validation
altogether.
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