WifiBackupRestore: Don't return null

The SettingsBackupAgent doesn't handle null returns, so return an empty
array if there is no data to backup.

Bug: 34831550
Test: Added unit test
Change-Id: Ieed471a9cc01ae58dcb70529f12974a76b2e8dbb
2 files changed
tree: 9e2777b1c972a69f1e65c7b32b9e86325c49fb97
  1. libwifi_hal/
  2. libwifi_system/
  3. service/
  4. tests/
  5. PREUPLOAD.cfg