zebra: fix FPM abort for unreach/prohibit routes

zebra: fix FPM abort for unreach/prohibit routes

b0e9567ed162da708f8d0b3a3caf87cd03b62e96 fixed an issue whereby
zebra would abort while building an update for a blackhole route.

The same issue, `assert(data_len)` failing in
`zfpm_build_route_updates()`, can be observed when building updates
for unreachable and prohibit routes.

To address this `netlink_route_info_fill()` is updated to not
indicate failure, due to lack of nexthops, for any blackhole routes.

Signed-off-by: Duncan Eastoe <duncan.eastoe@att.com>