[CTS] GetEnterpriseConfig should return null when unset Fix the CTS test. Bug: 162031037 Test: atest WifiNetworkSuggestionTest Merged-In: Id2b2f44981c7f98ec62f20cb9e8feb01a007b6b5 Change-Id: Id2b2f44981c7f98ec62f20cb9e8feb01a007b6b5 (cherry picked from commit ab2ec0749594f711dd4501ce01af26848f295a9a) (cherry picked from commit 92c6456738e99104c2130eface2ef211c120f82b)