If signal is lost, do not update the empty country iso for other slots

EmergencyNumberTracker shares the emergency country iso from multiple slots, it updates non-empty country iso for other slots if they don't have a network country, but it doesn't need / should not update empty country iso for other slots.

Test: atest EmergencyNumberTrackerTest
Bug: 179299344
Change-Id: I29efd0031f26fa94731b12d9865f1ab44a5ebd75