advapi32/tests: Simplify empty string check (PVS-Studio).

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
https://testbot.winehq.org/JobDetails.pl?Key=9984

Your paranoid android.


=== w7u (32 bit service) ===
The test timed out