Keywords: avoid warning at each message when no keywords are used

This fix does not apply when any keyword in configured which is already
harmless.

Written-by: Igor Almeida <igor.contato@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/Base.py

@@ -920,6 +920,9 @@ class BaseFolder(object):
 
         try:
             keywordmap = dstfolder.getrepository().getkeywordmap()
+            if keywordmap is None:
+                return selfflags
+
             knownkeywords = set(keywordmap.keys())
 
             selfkeywords = self.getmessagekeywords(uid)