msi: Simplify check for an empty string (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=10031

Your paranoid android.


=== w2000pro (32 bit action) ===
The test timed out

=== wxppro (32 bit action) ===
The test timed out

=== wvistau64 (32 bit action) ===
The test timed out

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

=== w8 (32 bit action) ===
The test timed out

=== wvistau64 (64 bit action) ===
Failure running script in VM: timed out waiting for the child process