demo: fix graph generation for empty prefixes

demo/demo

@@ -94,6 +94,7 @@ class Re6stNode(nemu.Node):
 
     def __init__(self, name, short):
         super().__init__()
+        print("Re6stNode ctor %r %r" % (name, short))
         self.name = name
         self.short = short
         self.Popen(('sysctl', '-q',
@@ -428,13 +429,12 @@ def route_svg(ipv4, z=4, default=type('', (), {'short': None})):
     graph: dict[Re6stNode, dict[tuple[Re6stNode, bool], list[Re6stNode]]] = {}
     for n in nodes:
         g = graph[n] = defaultdict(list)
-        g: dict[tuple[Re6stNode, bool], list[Re6stNode]]
-        for r in n.get_routes():
-            if (r.prefix and r.prefix.startswith('10.42.') if ipv4 else
-            r.prefix is None or r.prefix.startswith('2001:db8:')):
+        for route in n.get_routes():
+            if (route.prefix and route.prefix.startswith('10.42.') if ipv4 else
+                route.prefix is None or route.prefix.startswith('2001:db8:')):
                 try:
-                    g[node_by_ll(r.nexthop)].append(
-                        node_by_ll(r.prefix)[0] if r.prefix else default)
+                    if route.prefix:
+                        g[node_by_ll(route.nexthop)].append(node_by_ll(route.prefix)[0])
                 except KeyError:
                     pass
     gv = ["digraph { splines = true; edge[color=grey, labelangle=0];"]
@@ -442,20 +442,16 @@ def route_svg(ipv4, z=4, default=type('', (), {'short': None})):
     a = 2 * math.pi / N
     edges = set()
     for i, n in enumerate(nodes):
-        i: int
         gv.append('%s[pos="%s,%s!"];'
                   % (n.name, z * math.cos(a * i), z * math.sin(a * i)))
         l = []
         for p, r in graph[n].items():
-            p: tuple[Re6stNode, bool]
-            r: list[Re6stNode]
-            j: int = abs(nodes.index(p[0]) - i)
+            j = abs(nodes.index(p[0]) - i)
             l.append((min(j, N - j), p, r))
         for j, (_, (p2, t), r) in enumerate(sorted(l, key=lambda x: x[0])):
-            p2: Re6stNode
-            l2: list[str] = []
+            l2 = []
             arrowhead = 'none'
-            for r2 in sorted(r2.short for r2 in r):
+            for r2 in sorted(r2.short or '' for r2 in r):
                 if r2:
                     if r2 == p2.short:
                         r2 = '<font color="grey">%s</font>' % r2