I don’t really know how minecraft stores empty subchunks, but if they don’t have a specific tag like “IsEmpty“ / “hasNoBlocks“, they would need to check every single blocks of that subchunk, which if it is empty, would mean more calculus than just checking blocks on the path.
Chunk sections store a blockstate palette containing all block states present in the section. If this palette only contains a single entry, with that entry being air, the entire subchunk exclusively contains air blocks.
Alternatively, chunk sections stored in memory also seem to contain a counter for the ammount of non-air blocks.
MetaPixxl
Chunk sections store a blockstate palette containing all block states present in the section. If this palette only contains a single entry, with that entry being air, the entire subchunk exclusively contains air blocks.
Alternatively, chunk sections stored in memory also seem to contain a counter for the ammount of non-air blocks.