Block monotonicity checks can't be implemented for BTI row indexes
because they don't store full clustering positions, only some encoded
prefixes.
The emptiness check could be implemented with some effort,
but we currently don't bother.
The two tests which use this `is_empty()` method aren't very
useful anyway. (They check that the promoted index is empty when
there are no clustering keys. That doesn't really need a dedicated
test).
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