GetStringNameFromGuid() returns NULL for unknown GUID, instead of returning "UnknownDevice". The behavior change matches ShellProtocol.GetGuidName(). Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com> Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>