Fix NPE when deleting old zen rules

Test: manually revoke access for a DND app that has rules
created before and after b/189332346
Bug: 191773100
Bug: 189332346
Bug: 235823407

Change-Id: I069fcc124af24227fa50b61d4fc55d6dadc7a20b
Merged-In: I069fcc124af24227fa50b61d4fc55d6dadc7a20b
(cherry picked from commit b4fe101e)
(cherry picked from commit 7bb9cd92)
Merged-In: I069fcc124af24227fa50b61d4fc55d6dadc7a20b