range_tombstone_change_generator: flush: return early if empty
range_tombstone_change_generator: flush: return early if empty

Optimize the common, empty case.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

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