Allow for empty time suggestions in the TimeDetectorService. The empty suggestions are currently ignored but are likely to be used in future time detector strategy implementations. Bug: 140712361 Test: atest android.app.timedetector.PhoneTimeSuggestionTest Test: atest com.android.server.timedetector Change-Id: Ifb219702fa8bc53665a0d74752e06d851d86f8bb