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.

Fixes: 144610831

Change-Id: Idf9d4f08652ea91b26e89264b861091116c8df64
(cherry picked from commit 0922fdcdf011b9981829c95818aac8c296d3ed19)
1 file changed