fix: folder/*: never set self.messagelist to None

Empty the list by setting an empty dict.
Introduce BaseFolder().ismessagelistempty().

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

offlineimap/folder/Base.py

@@ -248,8 +248,17 @@ class BaseFolder(object):
 
         raise NotImplementedError
 
+    def ismessagelistempty(self):
+        """Empty everythings we know about messages."""
+
+        if len(self.messagelist.keys()) < 1:
+            return True
+        return False
+
     def dropmessagelistcache(self):
-        raise NotImplementedException
+        """Empty everythings we know about messages."""
+
+        self.messagelist = {}
 
     def getmessagelist(self):
         """Gets the current message list.