Zero-length files *should* be created by the store

[feross]

This is probably the issue, but it is not fully confirmed, as we didn't trace it exactly to this module.

In WebTorrent Desktop, there's a bug where a zero length file is not being created on the filesystem, which causes the mod-time code to think the file has been modified on disk and reverify the entire torrent.

Happens consistently with archive.org torrents.

cc @dcposch