One thing for sure
If there was no data worth mentioning on the phone, the FBI will never let us know anything about what they found "for national security reasons".
But you can be sure if there was anything on that phone, esp. anything dealing with another terror attack (say there's a text from someone saying "Yeah, and we're going to hit Brussels in March. We're holding up at 52 Main Strasse, Brussels. Don't tell anyone!") or they find something that they can, with 20/20 hindsight, link to any other "terror" group, we will hear about it over and over again. Probably through gov't leaks to certain media outlets at first. Or through to congress during meetings on banning encryption or something.