zebra: display seg6local only when specified

zebra: display seg6local only when specified

Srv6 routes which configure encap method, may not have
seg6local instructions. Generally speaking, seg6local
attributes that are not specified should not be dumped.

Before:
> B>* 10.200.0.0/24 [20/0] via fd00:125::2, ntfp2 (vrf default), label 16, seg6local unspec unknown(seg6local_context2str), seg6 2001:db8:1:1:1::, weight 1, 0\
0:00:17

After:
> B>* 10.200.0.0/24 [20/0] via fd00:125::2, ntfp2 (vrf default), label 16, seg6 2001:db8:1:1:1::, weight 1, 00:00:17

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