Only read payload signature if input file contains one am: c890e48d81 am: 9f1fe7c65a am: f63b92d996

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2514535

Change-Id: I588512513a981ac364be19c81d3eec626d95b0c6
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>