tests/sstable_assertions: Assert a promoted index is empty
tests/sstable_assertions: Assert a promoted index is empty

Signed-off-by: Duarte Nunes <a60e8b7496f7715d71354fe4c06dd24017025bf1@scylladb.com>

avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,duarten/scylla,duarten/scylla,duarten/scylla,scylladb/scylla