Allow PIC caches to cache nulls

Null-caching is opt-in; PIC caches that opt in should not be gated by
a system-wide flag.

Flag: EXEMPT bug-fix
Bug: 372923336
Test: atest
 * FrameworksCoreTests:PropertyInvalidatedCacheTests
 * FrameworksCoreTests:IpcDataCacheTest
 * CtsOsTestCases:IpcDataCacheTest
Change-Id: I87cad00378b9a665dd467753cdcfd38d4e6fb831