Filter out empty messages

It's not enough to check for None, we also need to check that there is
actually text in the message.

Signed-off-by: Anna Schumaker <Anna@OcarinaProject.net>

slack/threads.py

@@ -49,7 +49,7 @@ class Thread:
         # Read original message list
         o_list = []
         for message in self.do_fetch_messages(self.__last_ts):
-            if message["text"] != None:
+            if message["text"] != None and len(message["text"]) > 0:
                 o_list += [ chat.Message(message) ]
         o_list.sort()