don't add the empty string to additional nicks

thinko on my part, this was making the regex for matching all nicks to
'|nick' when the field is '', because of split producing ['']. in
particular this was making markov trigger on every line

markov/ircplugin.py

@@ -61,8 +61,11 @@ class Markov(Plugin):
     def handle_chatter(self, connection, event):
         """Learn from IRC chatter."""
         what = event.arguments[0]
-        all_nicks = '|'.join(connection.server_config.additional_addressed_nicks.split('\n') +
-                             [connection.get_nickname()])
+        if connection.server_config.additional_addressed_nicks:
+            all_nicks = '|'.join(connection.server_config.additional_addressed_nicks.split('\n') +
+                                 [connection.get_nickname()])
+        else:
+            all_nicks = connection.get_nickname()
         trimmed_what = re.sub(r'^(({nicks})[:,]|@({nicks}))\s+'.format(nicks=all_nicks), '', what)
         nick = irc.client.NickMask(event.source).nick
         target = reply_destination_for_event(event)