sstables: make delete_atomically() work with empty set
sstables: make delete_atomically() work with empty set

If delete_atomically() was called with a empty set for any reason,
it will fail to work because it relies on any of the sstables in
the set for getting the sstable directory.
This will be needed, in the future, when using sstable replacement
function only with new sstables.

Signed-off-by: Raphael S. Carvalho <1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>
Reviewed-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>
Message-Id: <20200305144657.9440-1-1e2942cea40e3b4d2162dfcd9e53e41c05a84d22@scylladb.com>

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