In case PcdFirmwareReleaseDateString is not set use a valid date
as fallback. Using "unknown" makes Windows unhappy.
Fixes: 4cb94f20b0
("OvmfPkg/SmbiosPlatformDxe: use PcdFirmware*")
Reported-by: ruifeng.gao@intel.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>