network/route-nexthop: make GatewayOnLink= support an empty string

And invalidate the route section if an invalid string is specified.

src/network/networkd-route-nexthop.c

@@ -124,14 +124,10 @@ int config_parse_route_gateway_onlink(
                 return 0;
         }
 
-        r = parse_boolean(rvalue);
-        if (r < 0) {
-                log_syntax(unit, LOG_WARNING, filename, line, r,
-                           "Could not parse %s=\"%s\", ignoring assignment: %m", lvalue, rvalue);
-                return 0;
-        }
-
-        route->gateway_onlink = r;
+        r = config_parse_tristate(unit, filename, line, section, section_line, lvalue, ltype, rvalue,
+                                  &route->gateway_onlink, network);
+        if (r <= 0)
+                return r;
 
         TAKE_PTR(route);
         return 0;

src/shared/conf-parser.c

@@ -1064,7 +1064,7 @@ int config_parse_tristate(
 
         if (isempty(rvalue)) {
                 *t = -1;
-                return 0;
+                return 1;
         }
 
         r = parse_tristate(rvalue, t);
@@ -1074,7 +1074,7 @@ int config_parse_tristate(
                 return 0;
         }
 
-        return 0;
+        return 1;
 }
 
 int config_parse_string(