network: set empty value for link-local route's gateway

When a route was created without gateway, after opening the connection editor,
that route's gateway was getting marked as error.

This commit changes the empty value on existing routes to
a valid "empty" IP.

Fixes GNOME/gnome-control-center#1055
9 jobs for network-ce-gateway-master in 2 minutes and 15 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed build #812735

00:01:24

 
  Test
passed test #812736

00:00:50

 
  Manual
manual asan #812738
privileged allowed to fail manual
manual flatpak #812737
allowed to fail manual
manual msan #812741
allowed to fail manual
manual tsan #812739
allowed to fail manual
manual ubsan #812740
allowed to fail manual
 
  Review
manual review #812742
allowed to fail manual
manual stop_review #812743
allowed to fail manual