Fix NPE in ComponentNameValidator

In general, we should consider null component names as invalid settings,
meaning that we don't restore.

b/79925290 to allow restoring null component names for specific
settings where null has semantic meaning.

Bug: 79910479
Test: 1) atest SettingsValidatorsTest
2) Manual:
- In Settings UI, select "None" for autofill service and accessibility
shortcut target
- "adb backup -keyvalue -f nullsettings.ab com.android.providers.settings"
- "adb restore nullsettings.ab" and verify no crashes
Change-Id: Iffecbe7d26a93a816e7be42f564ba471f9681876
(cherry picked from commit 5d26b8c0)