fix nil due date comparison bug in upcoming assignments
fix nil due date comparison bug in upcoming assignments

test plan
- create an assignment with no due date
- create a section override for the assignment with a due date
  less than one week in the future
- as a student not in the overridden section, ensure that pages
  that include the "Upcoming Assignment" list do not crash

Change-Id: I877fd8eabf31b68fb071903050ab3eb228050a70
Reviewed-on: https://gerrit.instructure.com/17520
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
QA-Review: Marc LeGendre <d33c80bc45d65303e33ca83108a9952b745af9ef@instructure.com>

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