Bug 918970 - display existing thread when sending a sms from contact app

Okay, confirmed with Steve, you're right that my case with sender===null for all outgoing messages is just frequent exception. So not easy to collapse, need to know incoming/outgoing. You can see how we detect outgoing\incoming (not just sent, received), here: https://github.com/azasypkin/gaia/blob/24fb519181335ff2fcf3a32cf1b775685f9089cf/apps/sms/js/thread_ui.js#L1574