Fixed issue with exception when IMAP transport returned no messages.

--HG--
branch : ashvetsov/fixes-to-push
extra : source : b9b283ff46d734933c4a715630e114d5e250a34b

django_mailbox/transports/imap.py

@@ -21,6 +21,10 @@ class ImapTransport(object):
 
     def get_message(self):
         typ, inbox = self.server.search(None, 'ALL')
+
+        if not inbox[0]:
+            return
+
         for key in inbox[0].split():
             try:
                 typ, msg_contents = self.server.fetch(key, '(RFC822)')