Summary:
- made `CreateCheckpoint` properly return `InvalidArgument` when called with an empty directory. Previously it triggered an assertion failure due to a bug in the logic.
- made `ldb` set empty `checkpoint_dir` if that's what the user specifies, so that we can use it to properly test `CreateCheckpoint` in the future.
Differential Revision: D6874562
fbshipit-source-id: dcc1bd41768261d9338987fa7711444289707ed7
1960e73e21