FindPackageHandleStandardArgs: improve error me...

...ssage when VERSION_VAR is empty or has been unset()

Currently when VERSION_VAR is set but empty,
find_package_handle_standard_args() emits a confusing error message,
saying that the package was found but of an unsuitable version "".

With this patch the error message correctly states that the package was
not found.

Fixes #23807
2 jobs for fphsa-error-message-empty-version-var