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