Don't cache empty IMAP folders

If a folder is empty, most servers will return EXISTS 0 and imaplib2
passes back ['0'] as return value to a select(). It returns [None] if
no EXISTS response was given by the server at all.

Attempting to fetch the UIDs of 0 emails which leads to
various error messages (One server responds with "NO No matching
messages", Gmail seems to say "BAD Bad message sequence 1:*" for some
(although it is working fine for me with Gmail, so it might behave
different for different people).

In case we get an None or 0 back, we simply stop caching messages as the
folder is empty. This should fix the various error reports that have
popped up.

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

offlineimap/folder/IMAP.py

@@ -120,7 +120,9 @@ class IMAPFolder(BaseFolder):
         imapobj = self.imapserver.acquireconnection()
         try:
             res_type, imapdata = imapobj.select(self.getfullname(), True)
-
+            if imapdata == [None] or imapdata[0] == '0':
+                # Empty folder, no need to populate message list
+                return
             # By default examine all UIDs in this folder
             msgsToFetch = '1:*'