Null checks on max/min call quality snapshots

The bad-quality snapshot may be null if the whole call had good quality.
The same is true of the good-quality snapshot if the whole call was bad.

Fixes: 130613973
Test: manual
Change-Id: Ibf7cbce46675a7998963070b776654033e0cc46d
Merged-In: Ibf7cbce46675a7998963070b776654033e0cc46d