SecurityPkg: Fix incorrect return value when File is NULL

The DxeTpmMeasureBootHandler and DxeTpm2MeasureBootHandler handlers
are SECURITY2_FILE_AUTHENTICATION_HANDLER prototype. This prototype
can not return EFI_INVALID_PARAMETER.

The prototype documentation states it returns EFI_ACCESS_DENIED if:

  "The file specified by File and FileBuffer did not authenticate,
   and the platform policy dictates that the DXE Foundation may not
   use File."

Correct the documentation, and add a early check, returning
EFI_ACCESS_DENIED when File is NULL.

Noticed while reviewing commit 6d575927.

Cc: Jiewen Yao <jiewen.yao@intel.com>
Cc: Jian J Wang <jian.j.wang@intel.com>
Cc: Chao Zhang <chao.b.zhang@intel.com>
Signed-off-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jian J Wang <jian.j.wang@intel.com>