Add missing check before calling PurgeObsoleteFiles in EnableFileDeletions (#5448)

Summary:
Calling PurgeObsoleteFiles with a JobContext for which HaveSomethingToDelete
is false is a precondition violation. This would trigger an assertion in debug builds;
however, in release builds with assertions disabled, this can result in the
pending_purge_obsolete_files_ counter in DBImpl underflowing, which in turn can lead
to the process hanging during database close.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5448

Differential Revision: D15792569

Pulled By: ltamasi

fbshipit-source-id: 82d92c9b4f6a9efcdc69dbb3d5a52a1ae2dd2472

db/db_filesnapshot.cc

@@ -57,7 +57,9 @@ Status DBImpl::EnableFileDeletions(bool force) {
   }
   if (file_deletion_enabled) {
     ROCKS_LOG_INFO(immutable_db_options_.info_log, "File Deletions Enabled");
+    if (job_context.HaveSomethingToDelete()) {
       PurgeObsoleteFiles(job_context);
+    }
   } else {
     ROCKS_LOG_WARN(immutable_db_options_.info_log,
                    "File Deletions Enable, but not really enabled. Counter: %d",