Update statistics only if needed.

Summary:
Update statistics only if needed.

Test Plan:

Reviewers:

CC:

Task ID: #

Blame Rev:

db/version_set.cc

@@ -2169,8 +2169,10 @@ Compaction* VersionSet::PickCompactionUniversal(int level, double score) {
   }
 
   // update statistics
+  if (options_->statistics != nullptr) {
     options_->statistics->measureTime(NUM_FILES_IN_SINGLE_COMPACTION,
                                       c->inputs_[0].size());
+  }
 
   c->input_version_ = current_;
   c->input_version_->Ref();