LocalStatus._folders needs to be {} not None

the LocalStatus._folders cache was changed to be a dict that can be
searched for names. One instance were _folders was set to "None" was
accidentally left over. Fix this.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/repository/LocalStatus.py

@@ -79,7 +79,7 @@ class LocalStatusRepository(BaseRepository):
         file.close()
         os.rename(filename + ".tmp", filename)
         # Invalidate the cache.
-        self._folders = None
+        self._folders = {}
 
     def getfolder(self, foldername):
         """Return the Folder() object for a foldername"""