Check settings validators for NPE

From b/79910479, we need to have better checks on settings validators.

Catch NPE for various settings validators and add a @Nullable annotation
to try to prevent NPEs in new validators.

Bug: 80067772
Test: atest SettingsValidatorsTest
Change-Id: I9a5058e1bae1ba0dea084c0589aa3e4ef3a51836
3 files changed