In many cases we trigger offstrategy compaction opportunistically
also when there's nothing to do. In this case we still print
to the log lots of info-level message and call
`run_offstrategy_compaction` that wastes more cpu cycles
on learning that it has nothing to do.
This change bails out early if the maintenance set is empty
and prints a "Skipping off-strategy compaction" message in debug
level instead.
Fixes#13466
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>
1134ca2767