KBFS: Mac OS keybase fs mkdir error

Using

`keybase fs mkdir pelorus32,anotheruser`

inside my Private directory, keybase throws the following error:

`▶ ERROR Cannot use empty name for BlockRef{id: 01c3cblahblah}`

But it creates the directory.

Is the directory that's been created a "functioning" and safe directory to use or is there a problem? What's the error about?

Start by reproducing `keybase fs mkdir pelorus32,anotheruser` inside a Private directory on Mac OS and capture the reported BlockRef error. Trace the filesystem operation that creates the directory and determine whether the resulting directory is usable and safe, and what causes the empty-name error.

Written by the indexing model from the issue text.