I'd been getting the unsent SMS every couple days on 4.4.2. I used 'Permission Friendly Apps' to find what had 'Send SMS' permissions. I don't use Skype from my phone, so I uninstalled it. I also disabled SMS and turned off the 'Improve Hangouts' option in Hangouts. After about a week, still no mysterious empty message.
It's not proof any of that fixes anything, but may be worth a try?
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