Return null instead of crashing when user does not exists

The checkIfKeyguardFeaturesDisabled function was crashing with a
nullptr exception when called on a userId that does not exists.
Return null instead. No user means nothing can be disabled.

Fix: 386971405
Test: atest \
SettingsLibRoboTests:com.android.settingslib.RestrictedLockUtilsTest
Flag: EXEMPT BUG_FIX
Change-Id: Ibc04b2faf8c0480d1f1b6bf4c7ee578b5b126d1a