isisd, tests: fix bug when sending TI-LFA repair paths to zebra

isisd, tests: fix bug when sending TI-LFA repair paths to zebra

Commit 4c75f7c7730220b fixed a bug in which the TI-LFA repair paths
weren't preserving the original Prefix-SID of the routes. That
commit, however, didn't update the zebra interface code to account
for backup nexthops that don't have a repair list but do have a
SR label. As a consequence, backup nexthops that didn't have any
repair label were not preserving the original Prefix-SID of the
corresponding routes. Fix this and update the TI-LFA topotest
accordingly.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>