libceph: allow addrvecs with a single NONE/blank address

Normally, an unused OSD id/slot is represented by an empty addrvec.
However, it also appears to be possible to generate an osdmap where
an unused OSD id/slot has an addrvec with a single blank address of
type NONE.  Allow such addrvecs and make the end result be exactly
the same as for the empty addrvec case -- leave addr intact.

Cc: stable@vger.kernel.org # 5.11+
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>