MMS: The send failed sms still exit in the thread when tap edit

The message if only have one message, and it is a send failed
message, click it and reedit it will delete the old message, but
if delete the old message the uri of conversation will be null,
so it can't query again, so the old message didn't be delete in
interface.

The mMsgListAdapter should change Cursor to null after delete
the only one message.

CRs-Fixed: 410738

Change-Id: Id9f08d33d16744b94ec8085e58fb393353366a0f