Subcompaction boundary keys should not terminate after an empty level

Summary: Now we skip to add boundary keys to subcompaction candidates since we see an empty level. This makes subcompaction almost disabled for universal compaction. We should consider all files instead.

Test Plan: Run existing tests.

Reviewers: IslamAbdelRahman, andrewkr, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D55005

db/compaction_job.cc

@@ -363,7 +363,7 @@ void CompactionJob::GenSubcompactionBoundaries() {
       size_t num_files = flevel->num_files;
 
       if (num_files == 0) {
-        break;
+        continue;
       }
 
       if (lvl == 0) {