zebra: Display nhg's afi as `No Afi`

zebra: Display nhg's afi as `No Afi`

When displaying `show nexthop-group rib` the afi
displayed for Nexthop Groups ( not singletons ) is
`bad-value` which while true to the specific of
the AFI it's not necessarily what we want to display
to the end operator.  Add a wrapper function for
nhg's to do the right thing.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>