Relax expected outcome of Sign_empty_key_name

Signing empty data is valid, and enforcing the expectation of
an error is incorrect. Updating the test to reflect that
the response from signing empty data is not important, though
the call returning is important.

Test: atest VtsHalWifiKeystoreV1_0TargetTest
Fixes: 140561000

Change-Id: Idf9d4f08652ea91b26e89264b861091116c8df64
1 file changed