Remove extraneous newline from ldb stderr (#6897)

Summary:
**Summary**
Remove the extraneous newline when using ldb tool. For example, the subcommand list_column_families will print an empty line to stderr even if there are no errors.

**Test plan**
Passed make check; manually tested a few ldb subcommands.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6897

Reviewed By: pdillinger

Differential Revision: D21819352

Pulled By: gg814

fbshipit-source-id: 5a16a6431bb96684fe97647f4d3ac5bf0ec7fc90

tools/ldb_tool.cc

@@ -124,7 +124,9 @@ int LDBCommandRunner::RunCommand(
 
   cmdObj->Run();
   LDBCommandExecuteResult ret = cmdObj->GetExecuteState();
+  if (!ret.ToString().empty()) {
     fprintf(stderr, "%s\n", ret.ToString().c_str());
+  }
   delete cmdObj;
 
   return ret.IsFailed() ? 1 : 0;