FileMetadataLoader skips empty directories

FileMetadataLoader has certain code paths like the one below which using listFiles API on the filesystem. This API ignores empty directories which is okay for non-transactional tables. However, in case of transactional table an empty base directory provides writeId information which is used to skip loading files which are not relevant for a given writeId. See AcidUtils#filterFilesForAcidState usage for details.