networkd: allow routes w/o gateway setting

networkd/templates/20-wired-routed.network.j2

@@ -19,7 +19,9 @@ IPForward=yes
 
 {% for route in network.routes|default([{'gateway': network.gateway}]) %}
 [Route]
+{% if route.gateway is defined %}
 Gateway={{ route.gateway }}
+{% endif %}
 {% if route.destination is defined %}
 Destination={{ route.destination }}
 {% endif %}