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

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

Change-Id: I5bd71682f98e8353eedc5a3118abd660cb3eaf41
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>