ignore null students when updating speedgrader select menu

For group assignments we might get back more submissions than we have
students loaded as only representative students are loaded for the
group. This resulted in a null student being passed to
updateSelectMenuStatus which errored out when asking the student for
its index. This patchset adds an early exit to updateSelectMenuStatus
in the case of a null student.

fixes GRADE-351

test plan:
 - Have a course with a teacher and some students
 - Create a group or two with multiple students in the groups
 - Create a group assignment
 - Go into speedgrader as a teacher
 - Grade the group assignment
 - Note that the status icon in the student menu reflects the graded
   status

Change-Id: Id1334311073633e8546a7de93feb5f55970ead63
Reviewed-on: https://gerrit.instructure.com/129442


Reviewed-by: Derek Bender <djbender@instructure.com>
Reviewed-by: Spencer Olson <solson@instructure.com>
Reviewed-by: Jeremy Neander <jneander@instructure.com>
Tested-by: Jenkins
QA-Review: Anju Reddy <areddy@instructure.com>
Product-Review: Keith T. Garner <kgarner@instructure.com>