fix editing due dates for courses which don't have a default section

Test Plan:
  For assignments, quizzes, and discussions:
  - Create a course without a default section
  - You should be able to save assignment overrides and due dates just
    fine.
  - Create a course with a default section
  - You should be able to save assignment overrides and due dates just
    fine.

fixes CNVS-5615

Change-Id: I6d98d0944b27e32b2eb7109acbdd126f5b231f64
Reviewed-on: https://gerrit.instructure.com/32856
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
QA-Review: Stanley Stuart <stanley@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>