MAP: GETMessagesLisitng include mandatory field always.

Pre condition:
Atleast one SMS is present in any of the folders
with no subject.

UseCase:
1. Connect MAP connection from MCE to DUT
2. Set folder to Telecom/msg/inbox
3. Execute MessageListing with the required data

Failure:
Mandatory field "subject" is not included for few entries
in messagesListing.

Fix:
For GETMessagesLisitng  ensure blank subject string is
set to include this mandatory feild and handle possible
"null" value fetched from database.

Change-Id: Ie29a61c83b22db9008184090a2f5b17d1f34b72b