IMAP.cachemessagelist(): Protect against empty folders

When invoked with FETCH 1:* (UID), imaplib returns [None] for empty
folders. We need to protect against this case and simply 'continue' here.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/IMAP.py

@@ -181,8 +181,10 @@ class IMAPFolder(BaseFolder):
         finally:
             self.imapserver.releaseconnection(imapobj)
         for messagestr in response:
-            # looks like: '1 (FLAGS (\\Seen Old) UID 4807)'
+            # looks like: '1 (FLAGS (\\Seen Old) UID 4807)' or None if no msg
             # Discard initial message number.
+            if messagestr == None:
+                continue
             messagestr = messagestr.split(' ', 1)[1]
             options = imaputil.flags2hash(messagestr)
             if not options.has_key('UID'):