Fixing `pijul add -r .`

This bug was introduced by a recent fix in libpijul::fs::add_inode, which was changed to return an error when trying to add an empty path

Created by  pmeunier  on September 14, 2021
RZ2FSIQ2ZOTTORLX432W7GWOTHQQ544ARFYI7NPNXSCWIK2RO4LAC
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