Add NPE check for screen and cellular update.

When battery is full of charge, usageList in BatteryStatsHelper
will be empty. As a result, we cannot find BatterySipper with
type screen and cell, which creates the null pointer error.

In this cl, I add NPE check for sipper. When sipper is null, all
data will be set to default value. I also add check for totalPower
so we won't have divided by zero error.

Bug: 35757789
Test: RunSettingsRoboTests
Change-Id: I80f3c0c542e0a50868e7c314a8d9b3c17999d8c6