CMake: Fix confusing FPHSA zstd version warning message

Previously if zstd was not found, one would see such a warning message

 Could NOT find WrapZSTD: Found unsuitable version "", but required is
 at least "1.3" (found ZSTD_LIBRARY-NOTFOUND)

This is because PC_ZSTD_VERSION was a defined variable with an empty
value, which makes FPHSA believe that a version was extracted from
somewhere.

Avoid passing that value directly.

Now the warning message is

 Could NOT find WrapZSTD (missing: ZSTD_LIBRARIES ZSTD_INCLUDE_DIRS)
 (Required is at least version "1.3")

Pick-to: 6.2 6.3 6.4
Change-Id: I88760d94db0d869d328085996298f4aaa88bc6c2
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

0 files changed, 0 insertions, 0 deletions