Master Storage malware verdict for a single file. Returned by
GetFileMalwareInfo. An empty / null pwszDescription means the file is either
clean OR the verdict is not available (vault has no SPE configured, file has no
master-storage ref, or Master Storage was unreachable). Clients should treat
"absence of description" as "no warning to show"; the actual security guarantee
is enforced at download time by DownloadDefenderHelper::RunCheck.