UserRestrictionUtils: Return null Bundle if there are no restrictions

* This prevents passing in an empty Bundle object when
  creating a new RestrictionSet from legacyLocalRestrictions
* This causes issues with the tag device_policy_restrictions, which
  is the only case where we actually create a RestrictionSet object

Change-Id: Ibc1ee8fd170836c3b34a2e3c2e6e24405bf957fe