fix assignment due date editing if there are no sections for the course
fix assignment due date editing if there are no sections for the course

test plan:
  - as an admin, create a course from the "Courses" tab in the admin
    settings for the account (you can get there by hovering over
    "Courses" in the top menu navigation, then clicking the account's
    name under "Managed Accounts."
  - Create an assignment for the course. Toggle "advanced options" on
    the assignment edit page. You should see the typical datepicker
    stuff there.
  - Save the assignment and make sure the dates get saved as usual.

fixes CNVS-5524

Change-Id: I88724bd745683473d9db494852a5982621b0c7f0
Reviewed-on: https://gerrit.instructure.com/20108
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Simon Williams <088e16a1019277b15d58faf0541e11910eb756f6@instructure.com>
QA-Review: Myller de Araujo <e5c7c5e1b7f2ce5e9870cc98ec52b20b3006cb48@instructure.com>
Product-Review: Stanley Stuart <2b43fb8b7a234825d50dd49ce7892d78a59da8f3@instructure.com>

samuels410/greatlakes-lms,Unow/ins,SwinburneOnline/canvas-lms,djbender/canvas-lms,kyroskoh/canvas-lms,landonwilkins/canvas-lms,juoni/canvas-lms,kyroskoh/canvas-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms,loganrun/pincan,Unow/canvas-lms,sfu/canvas-lms,roxolan/canvas-lms,venturehive/canvas-lms,arrivu/arrivu-lms,usmschuck/canvas,tgroshon/canvas-lms,tgroshon/canvas-lms,leftplusrightllc/canvas-lms,Jyaasa/canvas-lms,ShreniiNepal/shrenii_lmp,fronteerio/canvas-lms,jay3126/canvas-lms,shidao-fm/canvas-lms,wimemx/Canvas,whalejasmine/canvas-lms,va7map/canvas-lms,samuels410/greatlakes-lms,HotChalk/canvas-lms,djbender/canvas-lms,grahamb/canvas-lms,whalejasmine/canvas-lms,juoni/canvas-lms,samuels410/greatlakes-lms,wrdsb/canvas-lms,wye0220/shindig-canvas,juoni/canvas-lms,sli7246/canvas_branch,Unow/ins,arrivu/qibla-ecampus,leftplusrightllc/canvas-lms,grahamb/canvas-lms,Jyaasa/canvas-lms,jay3126/canvas-lms,Unow/canvas-lms,Rvor/canvas-lms,Unow/ins,kyroskoh/canvas-lms,venturehive/canvas-lms,Unow/canvas-lms,sdeleon28/canvas-lms,antoniuslin/canvas-lms,efrj/canvas-lms,redconfetti/canvas-lms,arrivu/qibla-ecampus,arrivu/jigsaw-lms,shidao-fm/canvas-lms,roxolan/canvas-lms,jay3126/canvas-lms,fronteerio/canvas-lms,tgroshon/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,arrivu/qibla-ecampus,va7map/canvas-lms,wimemx/Canvas,skmezanul/canvas-lms,usmschuck/canvas,wrdsb/canvas-lms,wrdsb/canvas-lms,redconfetti/canvas-lms,sfu/canvas-lms,sdeleon28/canvas-lms,fronteerio/canvas-lms,sfu/canvas-lms,utkarshx/canvas-lms,utkarshx/canvas-lms,arrivu/arrivu-lms,shidao-fm/canvas-lms,sdeleon28/canvas-lms,loganrun/pincan,arrivu/hoodemo,whalejasmine/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,Rvor/canvas-lms,landonwilkins/canvas-lms,sli7246/canvas_branch,arrivu/jigsaw-lms,efrj/canvas-lms,HotChalk/canvas-lms,HotChalk/canvas-lms,efrj/canvas-lms,roxolan/canvas-lms,antoniuslin/canvas-lms,ShreniiNepal/shrenii_lmp,greyhwndz/canvas-lms,grahamb/canvas-lms,wye0220/shindig-canvas,arrivu/jigsaw-lms,va7map/canvas-lms,Rvor/canvas-lms,fronteerio/canvas-lms,grahamb/canvas-lms,antoniuslin/canvas-lms,usmschuck/canvas,roxolan/canvas-lms,djbender/canvas-lms,arrivu/hoodemo,usmschuck/canvas,greyhwndz/canvas-lms,wye0220/shindig-canvas,skmezanul/canvas-lms,arrivu/hoodemo,loganrun/pincan,Jyaasa/canvas-lms,venturehive/canvas-lms,efrj/canvas-lms,sdeleon28/canvas-lms,juoni/canvas-lms,samuels410/greatlakes-lms,merichar/canvas-lms,djbender/canvas-lms,HotChalk/canvas-lms,landonwilkins/canvas-lms,whalejasmine/canvas-lms,Rvor/canvas-lms,va7map/canvas-lms,Jyaasa/canvas-lms,greyhwndz/canvas-lms,sli7246/canvas_branch,skmezanul/canvas-lms,jay3126/canvas-lms,wrdsb/canvas-lms,utkarshx/canvas-lms,arrivu/arrivu-lms,redconfetti/canvas-lms,SwinburneOnline/canvas-lms,kyroskoh/canvas-lms,landonwilkins/canvas-lms,skmezanul/canvas-lms,redconfetti/canvas-lms,tgroshon/canvas-lms,merichar/canvas-lms,ShreniiNepal/shrenii_lmp,venturehive/canvas-lms,antoniuslin/canvas-lms,sfu/canvas-lms,utkarshx/canvas-lms,leftplusrightllc/canvas-lms,merichar/canvas-lms,shidao-fm/canvas-lms,leftplusrightllc/canvas-lms,wimemx/Canvas,ShreniiNepal/shrenii_lmp,greyhwndz/canvas-lms