Add back NewEmptyIterator (#5203)

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

table/iterator.cc

@@ -157,6 +157,8 @@ class EmptyInternalIterator : public InternalIteratorBase<TValue> {
 };
 }  // namespace
 
+Iterator* NewEmptyIterator() { return new EmptyIterator(Status::OK()); }
+
 Iterator* NewErrorIterator(const Status& status) {
   return new EmptyIterator(status);
 }