Return empty set instead of null when user is not set.

- Avoid iterating over null set when checking components for user
  that isn't currently active.  This can happen if the user is changed,
  and the enabled component set is queried before the set has been
  rebuilt to include components for the new current user.

Bug: 30165350
Change-Id: I4dadad84e6bd2728bea5711b936cba91f6791c99