KeyMint VTS: catch empty cert chains

Explicitly detect empty cert chains returned by GenerateKey rather
than crashing when trying to dereference the first entry.

Bug: 195605180
Test: VtsAidlKeyMintTargetTest
Merged-In: Idad2703b458952ff599c6ccdd04a941aef7aedde
Change-Id: Idad2703b458952ff599c6ccdd04a941aef7aedde
Ignore-AOSP-First: already merged in aosp/master
1 file changed