util: Fix segfault when doing a backup with an empty backup_path
Revert change of 'util: Fix occurances where back() might be called on an
empty string' for zenfs_tool_backup().
Otherwise test/utils/0040_backup.sh segfaults when building ZenFS with
RocksDB 6.29.fb (c++11).
util: Fix segfault when doing a backup with an empty backup_path Revert change of 'util: Fix occurances where back() might be called on an empty string' for
zenfs_tool_backup()
.Otherwise test/utils/0040_backup.sh segfaults when building ZenFS with RocksDB 6.29.fb (c++11).