credstore: Never pass an empty challenge.

Bug: 156911917
Bug: 158107945
Test: atest VtsHalIdentityTargetTest
Test: atest android.security.identity.cts
Merged-In: Ia274fcbaabb0c0ec7c884070eb30737dc236c064
Change-Id: I51c450c1ac3f09ec1ce1d1e5eb43fe5c9eeb47c0
1 file changed