[res] Skip all actor logic when no actors are defined

Most devices don't define any actors, so the code that does
anything at all should only run if there's at least one named
actor defined - otherwise it would only waste CPU cycles

Test: build + boot + atest OverlayActorEnforcerTests
Flag: EXEMPT minor fix
Change-Id: Ibdac21607a755f478b0768dbb50a70063a2ce9c0