Avoid SIGSEGV if attest key generation fails.

Add an explicit check that `attested_key_cert_chain.size() > 0`.

Bug: 209672758
Test: atest 'PerInstance/AttestKeyTest#AllEcCurves/1_android_hardware_security_keymint_IKeyMintDevice_strongbox'

Signed-off-by: Brian J Murray <brianjmurray@google.com>
Change-Id: I4f7cb4fb1a30f26e6ef15c54714699f6ae91ba36