Fix error when we have got message

scripts/generate_chat_stat_daily.py

@@ -60,6 +60,9 @@ def main():
       labels[id] = begin.strftime("%x")
       print("\t{0}".format(labels[id]))
       
+      if messages is None:
+        continue
+
       resultMess = list(filter(filter_message, messages))
       length = len(resultMess)
       dataMess.append(length)

scripts/generate_chat_stat_monthly.py

@@ -60,6 +60,9 @@ def main():
       labels[id] = begin.strftime("%b %Y")
       print("\t{0}".format(labels[id]))
 
+      if messages is None:
+        continue
+      
       resultMess = list(filter(lambda mess: begin < datetime.strptime(mess["ts"], "%Y-%m-%dT%H:%M:%S.%fZ") < end, messages))
       length = len(resultMess)
       dataMess.append(length)