Allow for empty time suggestions

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