Bug 1025937 - Silently drop null intents in HealthReportBroadcastService.

It looks like an IntentService can sometimes be restarted with a null intent. The right thing to do appears to be to ignore it — we'll get another intent if we have to do some work. (If we were restarted mid-operation, we just wait until the next time.)
Why did you only do this for `HealthReportBroadcastService`? `AnnouncementsBroadcastService` uses a similar paradigm.