Don't create a file with empty favorites

A file could be created with empty favorites, preventing a late restore
(from D2D) from overwriting it.

Test: atest ControlsFavoritePersistenceWrapperTest
Test: manual D2d restore
Bug: 165200632
Change-Id: Ia99b8e98e4f62d8d0c7385c820c645f3aaabe6f3