Reduce empty SST creation/deletion during compaction (#4311)

Summary:
I have a PR to start calling `OnTableFileCreated` for empty SSTs: #4307. However, it is a behavior change so should not go into a patch release.

This PR adds back a check to make sure range deletions at least exist before starting file creation. This PR should be safe to backport to earlier versions.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4311

Differential Revision: D9493734

Pulled By: ajkr

fbshipit-source-id: f0d43cda4cfd904f133cfe3a6eb622f52a9ccbe8

HISTORY.md

@@ -5,6 +5,7 @@
 
 ### New Features
 ### Bug Fixes
+* Avoid creating empty SSTs and subsequently deleting them in certain cases during compaction.
 
 ## 5.16.0 (8/21/2018)
 ### Public API Change

db/compaction_job.cc

@@ -1077,7 +1077,8 @@ void CompactionJob::ProcessKeyValueCompaction(SubcompactionState* sub_compact) {
   }
 
   if (status.ok() && sub_compact->builder == nullptr &&
-      sub_compact->outputs.size() == 0) {
+      sub_compact->outputs.size() == 0 &&
+      !range_del_agg->IsEmpty()) {
     // handle subcompaction containing only range deletions
     status = OpenCompactionOutputFile(sub_compact);
   }