ART: Add null type to verifier

Add a null type to distinguish zero from null. This is an
infra-only change, as nothing will introduce null with this
commit, yet.

Bug: 22059710
Bug: 64683522
Bug: 69669661
Test: m test-art-host
Change-Id: Id2549a5aefefe9471d4bdbd2c8993395150947c6