vpn: allow IP configurations with routes and without addresses

Usually, when the method is "auto" we want to avoid configuring routes
until the automatic method completes. To achieve that, we clear the
"allow_routes_without_address" flag of l3cds when the method is "auto".

For VPNs, IP configurations with only routes are perfectly valid,
therefore set the flag.

src/core/vpn/nm-vpn-connection.c

@@ -1433,6 +1433,10 @@ _check_complete(NMVpnConnection *self, gboolean success)
     l3cd = nm_l3_config_data_new_from_connection(nm_netns_get_multi_idx(priv->netns),
                                                  nm_vpn_connection_get_ip_ifindex(self, TRUE),
                                                  connection);
+
+    nm_l3_config_data_set_allow_routes_without_address(l3cd, AF_INET, TRUE);
+    nm_l3_config_data_set_allow_routes_without_address(l3cd, AF_INET6, TRUE);
+
     _l3cfg_l3cd_set(self, L3CD_TYPE_STATIC, l3cd);
 
     _l3cfg_l3cd_gw_extern_update(self);