OnTableFileCreationCompleted use "(nil)" for empty file during flush (#5905)

Summary:
Compaction can call OnTableFileCreationCompleted(). If file is empty, "(nil)"
is used as the file name.
Do the same for flush.

Test plan (dev server):
```
make all
make check
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5905

Differential Revision: D17883285

Pulled By: riversand963

fbshipit-source-id: 6565884adbb00e8023d88b17dfb3b6eb92220b59

HISTORY.md

@@ -23,6 +23,7 @@
 * Added DisableManualCompaction/EnableManualCompaction to stop and resume manual compaction.
 * Add TryCatchUpWithPrimary() to StackableDB in non-LITE mode.
 * Add a new Env::LoadEnv() overloaded function to return a shared_ptr to Env.
+* Flush sets file name to "(nil)" for OnTableFileCreationCompleted() if the flush does not produce any L0. This can happen if the file is empty thus delete by RocksDB.
 ### Performance Improvements
 * Improve the speed of the MemTable Bloom filter, reducing the write overhead of enabling it by 1/3 to 1/2, with similar benefit to read performance.
 

db/builder.cc

@@ -243,6 +243,9 @@ Status BuildTable(
     env->DeleteFile(fname);
   }
 
+  if (meta->fd.GetFileSize() == 0) {
+    fname = "(nil)";
+  }
   // Output to event logger and fire events.
   EventHelpers::LogAndNotifyTableFileCreationFinished(
       event_logger, ioptions.listeners, dbname, column_family_name, fname,