zebra: fix FPM abort for unreach/prohibit routes

zebra: fix FPM abort for unreach/prohibit routes

f4517a795d9b719b220fa9013810c1a234e69568 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>
(cherry picked from commit 94f7786375030e08063cdae5b4577edf26adb456)