[automerger skipped] credstore: Never pass an empty challenge. am: bbb7f80377 -s ours

am skip reason: Change-Id Ia274fcbaabb0c0ec7c884070eb30737dc236c064 with SHA-1 673c6ced22 is in history

Original change: https://googleplex-android-review.googlesource.com/c/platform/system/security/+/11755141

Change-Id: I9adcbf7f0b5bb8fc8e6b18fee145277cd6891aba