Return default notification policy when calling from a user with no ZenModeConfig

Although this should be super rare, it can happen more often (with modes_multiuser) if we never got an onUserSwitched event for that user. Most callsites don't expect to get a null Policy, so even though null is technically allowed, returning _something_ is safer.

Bug: 323163267
Test: atest ZenModeHelperTest
Flag: android.app.modes_multiuser
Change-Id: Ie9ec86037f79bbc0bfe87679b7791979fdc2532d