Don't blow up if checking effective due dates for a non-assigned user
Don't blow up if checking effective due dates for a non-assigned user

Fixes CNVS-35631

Test plan:
* Create a course with an assignment and a student
* Create another course with another student
* In rails console:
  edd = EffectiveDueDates.for_course(course1)
  edd.in_closed_grading_period?(assignment, student_in_other_course)

This should return false, rather than crash.

Change-Id: Ib74c4371837dfc2152bf814eae839ead4b92d052
Reviewed-on: https://gerrit.instructure.com/105149
Tested-by: Neil Gupta <cc4fec8a34766005dd45d48be045c08d736c11ed@instructure.com>
Reviewed-by: Keith T. Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>
Reviewed-by: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>
QA-Review: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>
Product-Review: Keith T. Garner <a1a80143a840c140f31c3495ebbd9ec906e09fed@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,SwinburneOnline/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms