DB: use assertIsNotNone for unit test

Commit 48de2895b9a550a0944b31212349275605a4061d added a
validation that should have used assertIsNotNone.

TrivialFix

Change-Id: Ib6b82d90bb163f7ee06c916e52d3dd40ff2ba34d
Merge "DB: use assertIsNotNone for unit test"