Always pass a non-empty user string to `su`

Older versions of su would take "" as user, and map that to root. Newer
versions won't accept "". So let's use root explicitly, so that we keep
the existing behavior.

Closes: #904870

Signed-off-by: Antonio Terceiro <terceiro@debian.org>
Signed-off-by: Chris Hofstaedtler <zeha@debian.org>