MdeModulePkg/SecurityStubDxe: Mark the File parameter as OPTIONAL

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2652



According to the description, the File is optional and can be NULL

Signed-off-by: Guomin Jiang <guomin.jiang@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Hao A Wu <hao.a.wu@intel.com>
Cc: Dandan Bi <dandan.bi@intel.com>
Cc: Liming Gao <liming.gao@intel.com>

Reviewed-by: Jian J Wang <jian.j.wang@intel.com>