Slightly more sensible statistics log message when no data points have been collected

network.py

@@ -148,7 +148,7 @@ class PerfStatsStats:
                     (self.bytes_sent, self.bytes_received, self.bytes_tot,
                     self.keygens, self.sigs, self.verifs, self.dhs, self.N)
         else:
-            return "None"
+            return "N=0"
 
 
 class NetAddr: