Update trapezoid validation in battery chart view.

If the battery level data cannot be gotten for an hour, ag/19358207 changes the level indication from 0 to null. This cl updates the battery chart view trapezoid validation logic from checking the level not zero to not null.

Test: manual
Bug: 236101166
Change-Id: I240f2b02b94ba1a3d99022c7c72a7a5adb21f9a5