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

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

Change-Id: I64a437f5b65c8f5dbdc20c556c7f5bc457b6ddd7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>