Return empty suffix for invalid slots am: a6759d5d66 am: 2dba4ac8b4 am: 284508d0b2 am: 1016c8a7f2

Original change: https://android-review.googlesource.com/c/platform/hardware/interfaces/+/2166409

Change-Id: If20fbac9053aa475b47cb272cb35e67731d69eaa
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>