nixos/network-interfaces-systemd: do not ignore /0 gateway routes

While it might seem odd, 0.0.0.0/0 or ::/0 gateways are valid and
commonly used on point-to-point links (e.g. a wireguard tunnel) to
indicate that all traffic needs to be sent to a given interface.

systemd-networkd actually documents this as a valid configuration in its
man pages [1].

Tested to do the right thing in one of my NixOS containers using
a Wireguard tunnel as its default route.

[1] https://www.freedesktop.org/software/systemd/man/systemd.network.html#DefaultRouteOnDevice=

nixos/modules/tasks/network-interfaces-systemd.nix

@@ -134,7 +134,7 @@ in
               # Most of these route options have not been tested.
               # Please fix or report any mistakes you may find.
               routeConfig =
-                optionalAttrs (route.prefixLength > 0) {
+                optionalAttrs (route.address != null && route.prefixLength != null) {
                   Destination = "${route.address}/${toString route.prefixLength}";
                 } //
                 optionalAttrs (route.options ? fastopen_no_cookie) {