Add null-check for test variables in tearDown (@After)

In case the setup (@Before) never was able to fully run.
This can occur if there was an error in the setup.
Add this null check so that the NPE doesn't obscure the actual
root cause of the test failure.

Test: atest KeyguardIndicationTextViewTest
Test: atest KeyguardClockPositionAlgorithmTest
Fixes: 408471732
Fixes: 408470808
Flag: EXEMPT bugfix
Change-Id: I03da9238e30127451ff9891c62edaf651e4bbc6a