Summary:
#4905 removed the implementation of `NewEmptyIterator` but kept its
declaration in the public header. This breaks some systems that depend on
RocksDB if the systems use `NewEmptyIterator`. Therefore, add it back to fix. cc maysamyabandeh please remind me if I miss anything here. Thanks
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5203

Differential Revision: D14968382

Pulled By: riversand963

fbshipit-source-id: 5fb86e99c8cfaf9f7a9473cdb1355d7558ff6e01