Check if username is provided before trying plain authentication.

If the username value is not provided on the configuration file, then
username if of NoneType and NULL.join will bomb since Nones can't be
joined with strings.  So check if username is empty and raise an
exception if so.

Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/558
Signed-off-by: David Miguel Susano Pinto <carandraug+dev@gmail.com>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/imapserver.py

@@ -199,6 +199,11 @@ class IMAPServer(object):
           http://tools.ietf.org/html/rfc4616"""
 
         authc = self.username
+        if not authc:
+            raise OfflineImapError("No username provided for '%s'"
+                                    % self.repos.getname(),
+                                   OfflineImapError.ERROR.REPO)
+
         passwd = self.__getpassword()
         authz = b''
         if self.user_identity != None: