scripts/rxe_cfg_small: Print rxe-net devs only if list is non-empty

Otherwise it may cause false-positives, depending on how the output
is gathered, e.g. this:

  $ mapfile -t rxe_net < <(rxe_cfg_small.sh rxe-net)

will create rxe_net[@] with a single, empty item, fooling user that
there are some devices present even though they are not.

Change-Id: Ib2ce0fd9596b6eac1fc53054ef7eb7e94897a393
Signed-off-by: Michal Berger <michal.berger@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/19540


Reviewed-by: Karol Latecki <karol.latecki@intel.com>
Reviewed-by: Jim Harris <jim.harris@gmail.com>
Reviewed-by: Kamil Godzwon <kamilx.godzwon@intel.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>