SecurityPkg/DxeImageAuth: Mark the File parameter as OPTIONAL

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



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

Signed-off-by: Guomin Jiang <guomin.jiang@intel.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>

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