zebra: keep seg6local information from 'show ipv6 route' consistent with iproute2

zebra: keep seg6local information from 'show ipv6 route' consistent with iproute2

Srv6 nexthop segments may not be set when configuring seg6local
attributes. This is the case for the following seg6local route:

Dump in vtysh, extract from 'show ipv6 route'
> B>* 2001:db8:1:1:1::/128 [20/0] is directly connected, vrf1, seg6local End.DT46 table 10, seg6 ::, weight 1, 00:02:10

Dump in iproute2, extract from 'ip -6 route show'
> 2001:db8:1:1:1:: nhid 22  encap seg6local action End.DT46 vrftable 10 dev vrf1 proto bgp metric 20 pref medium

As can be seen, the 'seg6 ::' nexthop segment is not visible on iproute2,
because it is not set. Do not display seg6 ipv6 nexthop when not set.

After:
> B>* 2001:db8:1:1:1::/128 [20/0] is directly connected, vrf1, seg6local End.DT46 table 10, weight 1, 00:02:10

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>